drm/i915: convert DP autodither code to new infrastructure

The old code only handled either 6bpc or 8bpc. Since it's easy to do,
reorganize the code to be a bit more generic so that it can also handle
10bpc and 12bpc. Note that we still start with 8bpc, so there's no
functional change.

Also, since we no don't need to compute the 6BPC flag in the mode_valid
callback, we can consolidate things a bit. That requires though that
the link bw computation is moved up in the compute_config callback.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>