msm: kgsl: flush cache otherwise memory will be uninitialized

The memory is allocated by vmalloc_user which initializes the memory
to zero however the initialization was being discarded by the cache
invalidate before the memory was returned to userspace.

Change-Id: I4d44fb3c308bb04c6cb137f7d34cd38600980f42
Signed-off-by: Jeff Boody <jboody@codeaurora.org>