drm/i915: Kill has_dsi_encoder

has_dsi_encoder was introduced to indicate that the pipe is driving
a DSI encoder. Now that we have the output_types bitmask that can
tell us the same thing, let's just kill has_dsi_encoder.

v2: Rebase, handle BXT DSI transcoder, rewrote commit message

Cc: Jani Nikula <jani.nikula@intel.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/1466621833-5054-10-git-send-email-ville.syrjala@linux.intel.com