drm/i915: Integrate cmd parser kerneldoc

Ville noticed that we have this nice kerneldoc but it's not integrated
anywhere. Fix this asap!

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>