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
diff --git a/drivers/gpu/drm/i915/intel_dsi.c b/drivers/gpu/drm/i915/intel_dsi.c
index 3562bf3..1981212 100644
--- a/drivers/gpu/drm/i915/intel_dsi.c
+++ b/drivers/gpu/drm/i915/intel_dsi.c
@@ -268,6 +268,7 @@
 static bool intel_dsi_compute_config(struct intel_encoder *encoder,
 				     struct intel_crtc_state *pipe_config)
 {
+	struct drm_i915_private *dev_priv = encoder->base.dev->dev_private;
 	struct intel_dsi *intel_dsi = container_of(encoder, struct intel_dsi,
 						   base);
 	struct intel_connector *intel_connector = intel_dsi->attached_connector;
@@ -284,6 +285,14 @@
 	/* DSI uses short packets for sync events, so clear mode flags for DSI */
 	adjusted_mode->flags = 0;
 
+	if (IS_BROXTON(dev_priv)) {
+		/* Dual link goes to DSI transcoder A. */
+		if (intel_dsi->ports == BIT(PORT_C))
+			pipe_config->cpu_transcoder = TRANSCODER_DSI_C;
+		else
+			pipe_config->cpu_transcoder = TRANSCODER_DSI_A;
+	}
+
 	return true;
 }