msm: kgsl: Add uintptr_t cast for gpuaddr

gpuaddr in kgsl_memdesc is still an unsigned int. It needs to be
cast to a uintptr_t first and can then be cast to a pointer.

Change-Id: I1ff52dc2a747be53407a04efa153c6a6816acd82
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>