drm/i915/dp: Make intel_dp_output_format() usable for "4:2:0 also" modes

Hoist the drm_mode_is_420_only() from intel_dp_output_format()
into the caller. This will allow intel_dp_output_format() to be
reused for "4:2:0 also" modes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220322120015.28074-8-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>