drm: Add eDP 1.5 early transport definition

Add DP_PSR_ENABLE_SU_REGION_ET to enable panel early transport.

Cc: dri-devel@lists.freedesktop.org

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231218175004.52875-2-jouni.hogander@intel.com