drm/i915: clear DPLL reg when disabling i9xx dplls

Toghether with the hw state readout this should catch cases where we
don't properly updated the pll state (either in sw or hw). At least
for the shared dpll code the equivalent tricke helped a lot in
catching bugs.

Also rename the function prefix, it's not a generic piece of
infrastructure.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>