msm: kgsl: Add missing check for size against lower VA

_get_unmapped_area_topdown() subtracts the requested size
from lower entry base without really checking its value.

This leads to overflow while working at boundary conditions.

Add a condition to check entry base with the size and proceed.

Change-Id: Ic695da683b11de35c7c4b8936a35d693dc8fa452
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>