drm/i915: Move dpll crtc_mask and hw_state fields into separate struct

The new struct will be used in a follow up patch to allow a current and
a staged config to exist for the same shared DPLL.

v2: Rebase on by mask_to_refcount()->hweight32() change. (Damien)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>