drm/i915: Only touch WRPLL hw state in enable/disable hooks

To be able to do this we need to separately keep track of how many
crtcs need a given WRPLL and how many actually actively use it. The
common shared dpll framework already has all this, including massive
state readout and cross checking. Which allows us to do this switch in
a fairly small patch.

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