drm/i915/hdmi: Restructure 4:2:0 vs. 4:4:4 mode validation

drm/i915/hdmi: Restructure 4:2:0 vs. 4:4:4 mode validation

Restructure the HDMI mode validation to resemble the new
intel_hdmi_compute_formats(). Keeping the two in sync helps
to avoid different bugs in each.

The main difference between mode_valid() and
intel_hdmi_compute_formats() is that we don't want the
Hail Mary RGB fallback for "4:2:0 only" modes.

Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260409101539.22032-4-ville.syrjala@linux.intel.com
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>