drm/dp: add defines for downstream port types

Detailed cap info at address 80h is not available with DPCD ver
1.0. Whether such devices exist in the wild I don't know, but there
should be no harm done in having the defines for downstream port 0 in
address 05h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>