msm: kgsl: prevent multiple or partial mmaps of a buffer

These cases have never been a normal operation pattern of the
userspace driver. Sharing buffers through multiple mappings
is better left to dma-buf or ion.

Change-Id: I7e7658137937c96b9505d0f912dcb262d652e0c3
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>