drm/i915: enable/disable hooks for shared dplls

Looks at first like a bit of overkill, but
- Haswell actually wants different enable/disable functions for
  different plls.
- And once we have full dpll hw state tracking we can move the full
  register setup into the ->enable hook.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>