drm/i915/dp: Don't use DP link min bpp for the FRL link bandwidth check

drm/i915/dp: Don't use DP link min bpp for the FRL link bandwidth check

intel_dp_mode_min_link_bpp_x16() gives us the min bpp for the
DP link before the PCON, however intel_dp_mode_valid_downstream()
is trying to check for sufficient bandwidth on the HDMI FRL link
after the PCON. So the use of intel_dp_mode_min_link_bpp_x16() here
is incorrect.

Presumably even with FRL HDMI still can't go below 8bpc, so we should
just use that to give us the minimum required FRL bandwidth. And this
needs to account for the sink format (for 4:2:0 sub-sampling) since
that is what will be flowing over the HDMI link.

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260407183015.16256-1-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>