drm/i915: Remove encoder type checks from MST suspend/resume

Now that eDP encoders won't have can_mst==true, we can throw out
the encoder type checks from the MST suspend/resume paths.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466621833-5054-3-git-send-email-ville.syrjala@linux.intel.com