drm/crtc-helper: remove LOCKING from kerneldoc

- It yells.
- WARNing about incorrect locking is harder to ignore, so better
  than kerneldoc.
- Since those have been written per-crtc locks were added ...

So remove them and replace them by appropriate WARNs.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>