drm/i915/icl: Clear the shared port PLLs from the new crtc state

For consistency clear the icl_port_dplls from the new crtc state, when
releasing the DPLLs from the old crtc state. Leaving them set could
result in releasing the same PLLs multiple times from the same CRTC
state incorrectly (if the same CRTC was first used for a TypeC port then
for a combo PHY port).

Leaving the stale pointers behind happens not to cause a problem atm
(since the incorrect releasing will be a NOP), but we need to fix that
for consistency.

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/20190708140735.20198-2-imre.deak@intel.com