Add DRM_MODE_CONNECTOR_WRITEBACK to xf86drmMode.h

xf86drmMode.h currently duplicates the connector-type definitions from
drm_mode.h. Add DRM_MODE_CONNECTOR_WRITEBACK, which is only visible
through a client cap, from drm_mode.h.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>