drm/i915: implement new set_mode code flow

... using the pipe masks from the previous patch.

Well, not quite:
- We still need to call the disable_unused_functions helper, until
  we've moved the call to commit_output_state further down and
  adjusted intel_crtc_disable a bit. The next patch will do that.
- Because we don't support (yet) mode changes on more than one crtc at
  a time, some of the modeset_pipes checks are a bit hackish - but
  that only needs fixing once we incorporate global modeset support.

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