drm/i915: Kill dp_encoder_is_mst

dp_encoder_is_mst flag in the crtc state can be replaced by
intel_crtc_has_type(..., INTEL_OUTPUT_DP_MST). Let's do that.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479145447-12907-2-git-send-email-ville.syrjala@linux.intel.com