drm/msm/a6xx: Make GMU reset useful

Now that the GX domain is sorted we can wire up a working GMU reset.
IF a GMU hang was detected then try to forcefully shut down the GMU
in the power down sequence which should ensure that it can recover
normally on the next power up.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>