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>

drivers/gpu/drm/drm_crtc.c

@@ -227,6 +227,7 @@ static const struct drm_prop_enum_list drm_encoder_enum_list[] =
 	{ DRM_MODE_ENCODER_TVDAC, "TV" },
 	{ DRM_MODE_ENCODER_VIRTUAL, "Virtual" },
 	{ DRM_MODE_ENCODER_DSI, "DSI" },
+	{ DRM_MODE_ENCODER_DPMST, "DP MST" },
 };
 
 static const struct drm_prop_enum_list drm_subpixel_enum_list[] =

include/uapi/drm/drm_mode.h

@@ -181,6 +181,7 @@ struct drm_mode_get_plane_res {
 #define DRM_MODE_ENCODER_TVDAC	4
 #define DRM_MODE_ENCODER_VIRTUAL 5
 #define DRM_MODE_ENCODER_DSI	6
+#define DRM_MODE_ENCODER_DPMST	7
 
 struct drm_mode_get_encoder {
 	__u32 encoder_id;