msm: kgsl: Check the size while working at SVM boundary

_get_unmapped_area_topdown() tries to generate a new GPU VA
using lower entry in the tree. Sometimes the newly requested
size can be more than the lower entry VA and leads to overflow.

Check the size against the lower entry VA and return NOMEM if
size is more.

Change-Id: I81a7db2b6f4a91e78de8489f8da07192443a8b85
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>