drm/msm: rearrange the gpu_rmw() function

The register read-modify-write construct is generic enough
that it can be used by other subsystems as needed, create
a more generic rmw() function and have the gpu_rmw() use
this new function.

Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>