msm: kgsl: Change local vars being assigned from size_t to size_t

A size_t variable being assigned to an integer variable results in
truncation which is problematic. Fix this by changing all those to
size_t.

Change-Id: I144f9b5b862ab10e968e1422ed7a7e5d9566af6a
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>