drm/doc: Repleace LOCKING kerneldoc sections in drm_modes.c

There's not really any value in stating that no locking is needed. And
even if the comment is useful, a check for the right mutex at the
beginning of the function is better since that can't be ingored as
easily as a bit of documentation.

Note that drm_mode_probed_add in drm_crtc.c is also changed, the next
patch will move this into drm_modes.c

v2: Don't add locking WARN_ONs where it is not strictly required (i.e.
the two functions to validate/prune mode lists).

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