drm/msm/gpu: Snapshot GMU debug buffer

It appears to be a GMU fw build option whether it does anything with
debug and log buffers, but if they are all zeros it won't add anything
to the devcore size.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20211124214151.1427022-10-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>