drm/i915/cx0: Zero Cx0 PLL state before compute and HW readout

drm/i915/cx0: Zero Cx0 PLL state before compute and HW readout

Ensure Cx0 pll state is initialized to zero before any computation or HW
readouts, to prevent leaving some parameter in the state uninitialized
in the actual compute/HW readout functions later.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20251117104602.2363671-14-mika.kahola@intel.com