xf86drmMode.h: Add DisplayPort MST and DPI encoders/connectors

This brings xf86drmMode.h in sync with include/drm/drm_mode.h.
Eventually we really should only have a single set of definitions rather
than duplicating this in two files.

v2: add DPI encoder and connector types introduced in Linux v4.7

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>