drm/i915: introduce pipe_config->dither|pipe_bpp

We want to compute this earlier. To avoid a big complicated patch,
this patch here just does the big search&replace and still calls the
old functions at the same places.

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