drm/connector: Print connector colorspace in state debugfs

v3: Fix kerneldocs (kernel test robot)

v4: Avoid returning NULL from drm_get_colorspace_name

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Sebastian Wick <sebastian.wick@redhat.com>
Reviewed-by: Joshua Ashton <joshua@froggi.es>
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: Jani Nikula <jani.nikula@linux.intel.com>
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>