drm/i915: use sw tracked state to select shared dplls

Just yet another prep step to be able to do all this up-front, before
we've set up any of the shared dplls in the new state. This will
eventually be useful for atomic modesetting.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>