drm/i915/dp: Validate "4:2:0 also" modes twice

drm/i915/dp: Validate "4:2:0 also" modes twice

Currently we only validate "4:2:0 also" modes as RGB. But
if that fails we could perhaps still use the mode in with
4:2:0 output. All we have to do is retry the validation with
the different sink format.

So far we did the double validation only so far as it affects
PCON TMDS clock limits. But validating everything twice seems
a bit more sane.

Note that intel_dp_output_format() might still end up picking
RGB for the actual output format (and letting PCON deal with
the YCbCr conversion). So I suppose we could still fail the
validation due to that, and forcing even the output format
to 4:2:0 might solve it on a third try. But we'd need the
same fallback logic in intel_dp_compute_config(). For now
this seems sufficient.

v2: Pass output_format to intel_dp_mode_min_link_bpp_x16()
    Add TODO about remaining issues
    Nuke intel_dp_sink_format()

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/20260409101539.22032-6-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>