drm/i915: ->enable hook for WRPLLs

This time around another cute hack to pre-fill the pll->hw_state with
the right values. And also remove a bunch of checks which will be
replaced by lots more checks in the common framework.

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