drm/amd/display: Use plane->color_space for dpp if specified

[Why]
The input color space for the plane was previously ignored even if it
was set.

If a limited range YUV format was given to DC then the
wrong color transformation matrix was being used since DC assumed that
it was full range instead.

[How]
Respect the given color_space format for the plane if it isn't
COLOR_SPACE_UNKNOWN. Otherwise, use the implicit default since DM
didn't specify.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>