drm/amd/display: handle dp is usb-c

This patch adds handling of dp is usb-c, it is not tested but is
needed to support dp over usb-c

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>