msm: kgsl: Allow cache operations for an usermem mapped buffer

Allow one to perform cache operations on dma-buffers mapped from usermem
addresses.

CRs-Fixed: 699989
Change-Id: Ic0dedbad5152d838079c0f08757904714bfa796d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>