msm: kgsl: Ensure the GMU updates the wptr register

There is a chance that the GMU is powered down when KGSL tries to
update the ringbuffer wptr. If that happens, the wptr update will
be dropped. So if using the GMU, verify the update wasn't dropped.
If it was dropped, then repost the register write until it succeeds.

Change-Id: I08f900bf8689532ff0c994d29b5e20f2a211dfa4
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>