drm/i915: Reject mixing MST and SST/HDMI on the same digital port

drm/i915: Reject mixing MST and SST/HDMI on the same digital port

We can't mix MST with SST/HDMI on the same physical port, so we'll need
to reject such configurations in check_digital_port_conflicts(). Nothing
else will prevent this as MST has its fake encoders and its own connectors
so the cloning checks won't catch this.

The same digital port can be used multiple times, but only if all the
encoders involved are MST encoders, so we only want to check MST vs.
SST/HDMI, not MST vs. MST. And SST/HDMI vs. SST/HDMI we already check.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469717448-4297-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>