msm: kgsl: Correct page_count type in kgsl_pools struct

Update type of page_count variable in kgsl_pools struct
to unsigned int as it represents number of pages currently
present in the pool which can't be negative.

Change-Id: I3db83041cdcb862469df6f4fa87f40310ced7eb8
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>