drm/i915/mst: abstract choosing the MST mode to use

Clarify the conditions for choosing the MST mode to use by adding a new
function intel_dp_mst_mode_choose(). This also prepares for being able
to extend the MST modes to single-stream sideband messaging.

Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f626144f10b03d4609ff38a29bac013ecf3aca4e.1710839496.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_dp.c

@@ -4049,6 +4049,24 @@ static const char *intel_dp_mst_mode_str(enum drm_dp_mst_mode mst_mode)
 		return "SST";
 }
 
+static enum drm_dp_mst_mode
+intel_dp_mst_mode_choose(struct intel_dp *intel_dp,
+			 enum drm_dp_mst_mode sink_mst_mode)
+{
+	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
+
+	if (!i915->display.params.enable_dp_mst)
+		return DRM_DP_SST;
+
+	if (!intel_dp_mst_source_support(intel_dp))
+		return DRM_DP_SST;
+
+	if (sink_mst_mode == DRM_DP_SST_SIDEBAND_MSG)
+		return DRM_DP_SST;
+
+	return sink_mst_mode;
+}
+
 static enum drm_dp_mst_mode
 intel_dp_mst_detect(struct intel_dp *intel_dp)
 {
@@ -4059,12 +4077,7 @@ intel_dp_mst_detect(struct intel_dp *intel_dp)
 
 	sink_mst_mode = drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd);
 
-	if (i915->display.params.enable_dp_mst &&
-	    intel_dp_mst_source_support(intel_dp) &&
-	    sink_mst_mode == DRM_DP_MST)
-		mst_detect = DRM_DP_MST;
-	else
-		mst_detect = DRM_DP_SST;
+	mst_detect = intel_dp_mst_mode_choose(intel_dp, sink_mst_mode);
 
 	drm_dbg_kms(&i915->drm,
 		    "[ENCODER:%d:%s] MST support: port: %s, sink: %s, modparam: %s -> enable: %s\n",