drm/msm: Drop struct_mutex in submit path

It is sufficient to serialize on the submit queue now.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20210728010632.2633470-12-robdclark@gmail.com


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