drm/msm/kms: Use nested locking for crtc lock instead of custom classes

We don't need to make up custom lock classes here, we can simply use
mutex_lock_nested() and pass in the index of the crtc to the locking
APIs instead. This helps lockdep understand that these are really
different locks while avoiding having to allocate custom lockdep
classes.

Cc: Krishna Manikandan <mkrishn@codeaurora.org>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Fixes: b3d91800 ("drm/msm: Fix race condition in msm driver with async layer updates")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>