drm/i915: ->disable hook for WRPLLs

Currently still with a redudant WARN_ON in there, the common shared
dpll code will take care of this in the future.

Also we need to flip the switch for the transitional hack now to make
sure that we disable the right pll.

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