drm/amd/display: Add debugfs for testing output colorspace

In order to IGT test colorspace we'll want to print
the currently enabled colorspace on a stream. We add
a new debugfs to do so, using the same scheme as
current bpc reporting.

This might also come in handy when debugging display
issues.

v4:
- Fix function doc comment
- Fix sRGB debug print

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Joshua Ashton <joshua@froggi.es>

Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Sebastian Wick <sebastian.wick@redhat.com>
Cc: Vitaly.Prosyak@amd.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>