msm: kgsl: Pass correct buffer size for mapping gpuobj user memory

Current code incorrectly specifies buffer size as 0 for mapping
gpuobj user memory. This causes the map to fail because buffer
size is expected to be a non zero value. Fix this by passing the
correct size of the buffer to be mapped.

CRs-Fixed: 995378
Change-Id: I1a9aeb3f1dd67f014847322e5b14cba8775a82a4
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>