drm/i915/sdvo: Implement limited color range for SDVO HDMI properly

The SDVO/HDMI port register limited color range bit can only be used
with TMDS encoding and not SDVO encoding, ie. to be used only when
using the port as a HDMI port as opposed to a SDVO port. The SDVO
spec does have a note that some GMCHs might allow that, but gen4
bspec vehemently disagrees. I suppose on ILK+ it might work since
the color range handling is on the CPU side rather than on the PCH
side, so there is no clear linkage between the TMDS vs. SDVO
encoding and color range. Alas, I have no hardware to test that
theory.

To implement limited color range support for SDVO->HDMI we need to
ask the SDVO device to do the range compression. Do so, but first
check if the device even supports the colorimetry selection.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200108181242.13650-5-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>