drm/connector: Add enum documentation to drm_colorspace

To match the other enums, and add more information about these values.

v2:
 - Specify where an enum entry comes from
 - Clarify DEFAULT and NO_DATA behavior
 - BT.2020 CYCC is "constant luminance"
 - correct type for BT.601

v4:
- drop DP/HDMI clarifications that might create
  more questions than answers

v5:
- Add note on YCC and RGB variants

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Sebastian Wick <sebastian.wick@redhat.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Simon Ser <contact@emersion.fr>

Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Sebastian Wick <sebastian.wick@redhat.com>
Cc: Vitaly.Prosyak@amd.com
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Joshua Ashton <joshua@froggi.es>
Cc: Simon Ser <contact@emersion.fr>
Cc: Melissa Wen <mwen@igalia.com>
Cc: dri-devel@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>