drm/i915: Avoid mixing up SST and MST in DDI setup

drm/i915: Avoid mixing up SST and MST in DDI setup

The MST vs. SST selection should depend purely on the choice of the
connector/encoder. So don't try to determine the correct DDI mode
based on the intel_dp->is_mst, which simply tells us whether the sink
is in MST mode or not. Instead derive the information from the encoder
type. Since the link training code deals in non-fake encoders, we'll
also need to keep a second copy of that information around, which we'll
now designate as 'link_mst'.

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/1469717448-4297-4-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>