drm/i915/mst: Document the userspace fail with possible_crtcs

To avoid accidentally breaking things in the future add a
comment explaining why we misconfigure the pipe_mask.

Also toss in a TODO for investigating a single encoder
approach as opposed to the encoder-per-pipe approach.

v2: Drop a bogus TODO comment
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002162505.30716-6-ville.syrjala@linux.intel.comReviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>

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

@@ -615,6 +615,14 @@ intel_dp_create_fake_mst_encoder(struct intel_digital_port *intel_dig_port, enum
 	intel_encoder->power_domain = intel_dig_port->base.power_domain;
 	intel_encoder->port = intel_dig_port->base.port;
 	intel_encoder->cloneable = 0;
+	/*
+	 * This is wrong, but broken userspace uses the intersection
+	 * of possible_crtcs of all the encoders of a given connector
+	 * to figure out which crtcs can drive said connector. What
+	 * should be used instead is the union of possible_crtcs.
+	 * To keep such userspace functioning we must misconfigure
+	 * this to make sure the intersection is not empty :(
+	 */
 	intel_encoder->pipe_mask = ~0;
 
 	intel_encoder->compute_config = intel_dp_mst_compute_config;