drm/nouveau: Replace the iturbt_709 prop with the standard COLOR_ENCODING prop

drm/nouveau: Replace the iturbt_709 prop with the standard COLOR_ENCODING prop

Replace the ad-hoc iturbt_709 property with the new standard
COLOR_ENCODING property. Compiles, but not tested.

v2: Fix typos (Ilia)

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: nouveau@lists.freedesktop.org
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220134816.15229-1-ville.syrjala@linux.intel.com
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Ben Skeggs <bskeggs@redhat.com> #irc