drm/i915: Clear the shared PLL from the put_dplls() hook

For symmetry with the get_dplls() hook which sets the shared_dpll
pointer clear the same pointer from the put_dplls() hook.

While at it also constify the old crtc state.

v2:
- Constify the old crtc state. (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708140735.20198-1-imre.deak@intel.com