drm/i915: Allow MST sinks to work even if drm_probe_ddc() fails

With HSW + Dell UP2414Q (at least) drm_probe_ddc() occasionally fails,
and then we'll assume that the entire display has been disconnected.
We don't need the EDID from the main link, so we can simply check if
the sink is MST capable, and if so treat is as connected.

v2: Skip drm_probe_ddc() entirely for MST (Daniel)

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Cc: Manasi D Navare <manasi.d.navare@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1469800276-6979-1-git-send-email-ville.syrjala@linux.intel.com