[Why]
To prepare for the upcoming DRM plane color management properties
we need to correct a lot of wrong behavior and assumptions made for
CRTC color management.

The documentation added by this commit in amdgpu_dm_color explains
how the HW color pipeline works and its limitations with the DRM
interface.

The current implementation does the following wrong:
- Implicit sRGB DGM when no CRTC DGM is set
- Implicit sRGB RGM when no CRTC RGM is set
- No way to specify a non-linear DGM matrix that produces correct output
- No way to specify a correct RGM when a linear DGM is used

We had workarounds for passing kms_color tests but not all of the
behavior we had wrong was covered by these tests (especially when
it comes to non-linear DGM). Testing both DGM and RGM at the same time
isn't something kms_color tests well either.

[How]
The specifics for how color management works in AMDGPU and the new
behavior can be found by reading the documentation added to
amdgpu_dm_color.c from this patch.

All of the incorrect cases from the old implementation have been
addressed for the atomic interface, but there still a few TODOs for
the legacy one.

Note: this does cause regressions for kms_color@pipe-a-ctm-* over HDMI.

The result looks correct from visual inspection but the CRC no longer
matches. For reference, the test was previously doing the following:

linear degamma -> CTM -> sRGB regamma -> RGB to YUV (709) -> ...

Now the test is doing:

linear degamma -> CTM -> linear regamma -> RGB to YUV (709) -> ...
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>