drm/msm: Clean up GMU OOB set/clear handling.

Now that the bug is fixed in the minimal way for stable, go make the
code table-driven.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>