drm/edid: add drm_edid_is_digital()

Checking edid->input & DRM_EDID_INPUT_DIGITAL is common enough to
deserve a helper that also lets us abstract the raw EDID a bit better.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/4bdb407bf189fd922be022eb2f9564692377c81d.1692884619.git.jani.nikula@intel.com