drm/i915/hdmi: Introduce intel_hdmi_is_ycbr420()

drm/i915/hdmi: Introduce intel_hdmi_is_ycbr420()

Introduce a small helper which given the crtc state tells us
whether we're output YCbCr 4:2:0 or not. For native HDMI this
is rather simple as we just look at the output_format. But I
think the helper is beneficial since with DP HDMI DFPs we're
going to need a more complex variant, and I want to unify the
DP and HDMI sides of that as much as possible.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015133921.4609-3-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>