drm/i915: Use the correct bpp when validating "4:2:0 only" modes

drm/i915: Use the correct bpp when validating "4:2:0 only" modes

When validating a "YCbCr 4:2:0 only" mode we must take into
account the fact that we're going to be outputting YCbCr
4:2:0 or 4:4:4 (when a DP->HDMI protocol converter is doing
the 4:2:0 downsampling). For YCbCr 4:4:4 the minimum output
bpc is 8, for YCbCr 4:2:0 it'll be half that. The currently
hardcoded 6bpc is only correct for RGB 4:4:4, which we will
never use with these kinds of modes. Figure out what we're
going to output and use the correct min bpp value to validate
whether the link has sufficient bandwidth.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917214335.3569-3-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>