drm/i915/display: Fixes quantization range for YCbCr output

This patch fixes the quantization range for YCbCr output on
Lspcon based devices.

v2: Re-phrased the description and added Ville's Rb.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130204738.2443-5-uma.shankar@intel.com