drm/i915: Track active streams also for DP SST

s/active_mst_links/active_streams/ and use it also for SST. We can then
use this information in the hpd handling to see if the link is active
or not, and thus whether we may need to retrain.

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>
Link: http://patchwork.freedesktop.org/patch/msgid/1469717448-4297-6-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>