drm/amd/display: port to using drm dpcd defines

We only keep one list of these defines in the kernel, so we should use it.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>