drm/msm: Remove unnecessary struct_mutex

The struct_mutex locking is a remnant from the days before per-obj locks,
and no longer needed.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20211109181117.591148-2-robdclark@gmail.com


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