drm/edid: Extract drm_edid_decode_mfg_id()

Make the PNPID decoding available for other users.

Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220510104242.6099-15-ville.syrjala@linux.intel.com