drm/i915: Fixup dp mst encoder selection

In

commit 8c7b5ccb


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>