drm/panelreplay: dpcd register definition for panelreplay

Add DPCD register definition for discovering, enabling and
checking status of panel replay of the sink.

Cc: Jouni Högander <jouni.hogander@intel.com>
Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231108072303.3414118-2-animesh.manna@intel.com