msm: kgsl: Detect gpu mem entry collisions when using MAP_FIXED flag

When using MAP_FIXED flags, detect collisions with previous gpu memory
entries. Fail mmap() if previous gpu entries exist.

Change-Id: I09f456677e7fc81b8bdf1b2f5724569220c7e897
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>