drm/i915/dp: Require a HDMI sink for YCbCr output via PCON

drm/i915/dp: Require a HDMI sink for YCbCr output via PCON

DVI sinks can't deal with YCbCr. Make sure we have a HDMI sink connected
after the PCON before doing YCbCr 4:2:0 output.

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/20260409101539.22032-7-ville.syrjala@linux.intel.com
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>