FIELD OF INVENTION The invention relates to controlled release dosage form comprising an atypical antipsychotic agent. More particularly, the invention relates to controlled release dosage form comprising paliperidone or its pharmaceutically acceptable salts thereof and the process for preparing the same. BACKGROUND OF THE INVENTION AND RELATED PRIOR ART Paliperidone is known as a major active metabolite of risperidone. It is chemically known as (±)-3-[2-[4- (6-fluoro-l, 2-benzisoxazol-3-yl)-l-piperidinyl] ethyl] -6, 7, 8, 9 -tetra hydro-9-hydroxy-2- methyl-4Hpyrido [1, 2-a] pyrimidin-4-one. It basically comprises a mixture of both (R) and (S) forms of risperidone and is disclosed in US 4,804,663 and 5,158,952. Paliperidone is marketed under the trade name Invega in the form of extended release tablet in the U.S. and Europe for the treatment of acute (short-term) as well as maintenance (long-term) treatment of schizophrenia. Paliperidone has a long half-life - about one day, so it is not a suitable drug for extended delivery. However, side effects such as anxiety, somnolence, dizziness, constipation, extra-pyramidal symptoms may be related to high blood plasma concentration levels restricting the ability to administer a single daily immediate release dose. Many efforts have been taken to control the release of the drug to provide extended release profile, while trying to control possible side-effects. A number of design options are available to control or modulate the drug release from a dosage form, but osmotic system of delivery is found to be most preferred design to control the release of drug from dosage form as it is independent of pH, presence of food and other physiological factors with respect to other conventional controlled release system. Commercially available Invega® is a capsule shaped tablet which consists of an osmotically active trilayer core surrounded by a subcoat and a semi¬permeable membrane. The trilayer core is composed of two drug layers containing the drug and excipients, and a push layer containing osmotically active components. There are two precision laser-drilled orifices on the drug-layer dome of the tablet. Patents / Patent applications, which disclose various osmotic controlled release compositions comprising paliperidone, are disclosed below: U.S. 2004/0092534, U.S. 2005/0208132, U.S.2005/0232995, U.S. 2006/0189635, U.S. 2009/0202631 and U.S. 2009/0227605 disclose an oral dosage form wherein said dosage form comprises a dose D of a benzisoxazole and an oral sustained release dosing structure adapted to sustainably release the benzisoxazole derivative at rates that provide (a) a mean, single dose, maximum plasma concentration Cmax of the benzisoxazole derivative and pharmacologically active metabolites thereof taken together and (b) a mean, single dose, area under a plasma concentration-time curve for AUQnf of the benzisoxazole derivative and pharmacologically active metabolites thereof taken together, which satisfy the relationship about 0.5 ng/mL/mg