drm/i915: add has_dsi_encoder to crtc state

DSI has quite a few special cases, like DP, so add it to crtc
state. This way we can get rid of a number of intel_pipe_has_type()
checks for DSI. This isn't necessarily the prettiest way, but it's a
step towards being aligned with what's being done with other encoders.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448619706-21293-3-git-send-email-jani.nikula@intel.com