drm/i915: extract adjusted mode computation

While at it, adjust a few things:
- Only assigng the new mode to crtc->mode right before calling the
  mode_set callbacks - none of the previous callbacks depend upon
  this, they all use the mode argument (as they should).
- Check encoder->new_crtc instead of the current crtc to check whether
  the encoder will be used. This prepares for moving the staged output
  committing further down in the sequence. Follow-on patches will fix
  up individual ->mode_fixup callbacks (only tv and lvds are affected
  though).

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