drm/i915: Extract intel_dp_output_format()

Refactor the output_format calculation into a helper so that
we can reuse it for mode validation as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917214335.3569-1-ville.syrjala@linux.intel.com


Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>