drm/msm: Remove struct_mutex usage

The remaining struct_mutex usage is just to serialize various gpu
related things (submit/retire/recover/fault/etc), so replace
struct_mutex with gpu->lock.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20211109181117.591148-4-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>