drm/i915/display: use drm_edid_is_digital()

Reduce the use of struct edid and drm_edid_raw().

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dbc0269d34f3140aff410eefae8a2711c59299b3.1692884619.git.jani.nikula@intel.com