drm/msm: Drop struct_mutex in shrinker path

Now that the inactive_list is protected by mm_lock, and everything
else on per-obj basis is protected by obj->lock, we no longer depend
on struct_mutex.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>