Only used by the lvds encoder. Note that we shouldn't do the same
simple conversion with the FORCE_6BPC flag, since that's much better
handled by moving all the pipe_bpc computation around.

This requires that we pass the pipe config around to encoders, so
that they can set special attributes and set constraints. To do so
introduce a new ->compute_config encoder callback, which is called in
stead of the drm crtc helper's ->mode_fixup.

To avoid massive churn all over the codebase we don't want to convert
all existing ->mode_fixup functions. Instead I've opted to convert
them on an as-needed basis (mostly to cut down on rebase conflicts and
to have more freedom to experiment around while developing the
patches).
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>