drm: add DP MST encoder type

This adds an encoder type for DP MST encoders.

Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/drm_crtc.c
Simple merge

diff --cc include/uapi/drm/drm_mode.h
Simple merge