drm/uapi: Document CTM matrix better

Document in which order the CTM matrix elements are stored.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230411222931.15127-2-ville.syrjala@linux.intel.com


Reviewed-by: Xaver Hugl <xaver.hugl@gmail.com>
Acked-by: Simon Ser <contact@emersion.fr>