drm/i915: Fixup dp mst encoder selection

In

commit 8c7b5ccb729870e606321b3703e2c2e698c49a95
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Tue Apr 21 17:13:19 2015 +0300

    drm/i915: Use atomic helpers for computing changed flags

we've switched over to the atomic version to compute the
crtc->encoder->connector routing from the i915 variant. That one
relies upon the ->best_encoder callback, but the i915-private version
relied upon intel_find_encoder. Which didn't matter except for dp mst,
where the encoder depends upon the selected crtc.

Fix this functional bug by implemented a correct atomic-state based
encoder selector for dp mst.

Note that we can't get rid of the legacy best_encoder callback since
the fbdev emulation uses that still. That means it's incorrect there
still, but that's been the case ever since i915 dp mst support was
merged so not a regression. Best to fix that by converting fbdev over
to atomic too.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -357,6 +357,16 @@ intel_dp_mst_mode_valid(struct drm_connector *connector,
 	return MODE_OK;
 }
 
+static struct drm_encoder *intel_mst_atomic_best_encoder(struct drm_connector *connector,
+							 struct drm_connector_state *state)
+{
+	struct intel_connector *intel_connector = to_intel_connector(connector);
+	struct intel_dp *intel_dp = intel_connector->mst_port;
+	struct intel_crtc *crtc = to_intel_crtc(state->crtc);
+
+	return &intel_dp->mst_encoders[crtc->pipe]->base.base;
+}
+
 static struct drm_encoder *intel_mst_best_encoder(struct drm_connector *connector)
 {
 	struct intel_connector *intel_connector = to_intel_connector(connector);
@@ -367,6 +377,7 @@ static struct drm_encoder *intel_mst_best_encoder(struct drm_connector *connecto
 static const struct drm_connector_helper_funcs intel_dp_mst_connector_helper_funcs = {
 	.get_modes = intel_dp_mst_get_modes,
 	.mode_valid = intel_dp_mst_mode_valid,
+	.atomic_best_encoder = intel_mst_atomic_best_encoder,
 	.best_encoder = intel_mst_best_encoder,
 };