drm: Move drm_display_mode an related docs into kerneldoc

This was in the documentation for modeset helper hooks, where it is a
bit misplaced.

v2: Reindent the drm_mode_status enum, inspired by Ville.

v3: Suggestions from Ville and Thierry.

v4: Small fixup that 0day spotted.

v5: Slight change to avoid accidental headings in kerneldoc output.

Cc: ville.syrjala@linux.intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-27-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v3)
Reviewed-by: Thierry Reding <treding@nvidia.com> (v3)