msm: kgsl: Fix num_pages calculation while shrinking pool

Correct num_pages calculation to ensure shrinker is able
to free pages from higher order pools and also make sure
shrinker is not able to free pages from pool beyond
reserved pages.

Change-Id: Iedefe74caca7d04bf4b8e25915e8470929688424
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>