drm/i915/bxt: add dsi transcoders

The BXT display connections have DSI transcoders A and C that can be
muxed to any pipe, not unlike the eDP transcoder. Add the notion of DSI
transcoders.

The "normal" transcoders A, B and C are not used with BXT DSI, so care
must be taken to avoid accessing those registers with DSI transcoders in
the hardware state readout, modeset, and generally everywhere.

v2: addressing comments by Ville:
 - rename the dsi get config function to hsw_get_dsi_transcoder_state
 - rebase onto the higher level split of pipe/transcoder functions
 - use more has_dsi_encoder as we can now because of the above,
   with no need to look at the transcoder so much
 - rename IS_DSI_TRANSCODER to transcoder_is_dsi
 - use the above a bit more instead of comparing to < TRANSCODER_EDP

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/299740536b7941e31b2744f3ce34f7afe936a771.1458313400.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -437,7 +437,8 @@ struct intel_crtc_state {
 	bool has_infoframe;
 
 	/* CPU Transcoder for the pipe. Currently this can only differ from the
-	 * pipe on Haswell (where we have a special eDP transcoder). */
+	 * pipe on Haswell and later (where we have a special eDP transcoder)
+	 * and Broxton (where we have special DSI transcoders). */
 	enum transcoder cpu_transcoder;
 
 	/*