drm/amd/display: Calculate output_color_space after pixel encoding adjustment

Code in get_output_color_space depends on knowing the pixel encoding to
determine whether to pick between eg. COLOR_SPACE_SRGB or
COLOR_SPACE_YCBCR709 for transparent RGB -> YCbCr 4:4:4 in the driver.

v2: Fixed patch being accidentally based on a personal feature branch, oops!

Fixes: ea117312 ("drm/amd/display: Reduce HDMI pixel encoding if max clock is exceeded")
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org