drm/msm/gpu: Make a6xx_get_gmu_log() more generic

Turn it into a thing we can use to snapshot other GMU buffers.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20211124214151.1427022-8-robdclark@gmail.com


Signed-off-by: Rob Clark <robdclark@chromium.org>