drm/i915/hdmi: Unify "4:2:0 also" logic between .mode_valid() and .compute_config()

Currently .mode_valid() and .compute_config() have their "4:2:0 also"
logic inverted. Unify things so that we use the same logic on both
sides.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015133921.4609-5-ville.syrjala@linux.intel.com


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