drm/i915: Kill has_dp_encoder from pipe_config

Use the new output_types bitmask instead of has_dp_encoder.
To make it less oainlful provide a small helper
(intel_crtc_has_dp_encoder()) to do the bitsy stuff.

v2: Rebase

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/1466621833-5054-7-git-send-email-ville.syrjala@linux.intel.com