drm/i915/hdmi: Extract intel_hdmi_output_format()

Reorganize the HDMI 4:2:0 handling a bit by introducing
intel_hdmi_output_format(). We already have the DP counterpart
and I want to unify the 4:2:0 handling across both a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015133921.4609-6-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>