drm/i915/cx0: Sanitize setting the Cx0 PLL use_c10 flag

drm/i915/cx0: Sanitize setting the Cx0 PLL use_c10 flag

Sanitize setting the Cx0 PLL use_c10 flag during state computation and
HW readout, making sure they happen the same way in the
intel_c{10,20}pll_calc_state() and intel_c{10,20}pll_readout_hw_state()
functions.

Follow-up changes will add more state computation/HW readout, this
change prepares for those as well.

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/20251117104602.2363671-4-mika.kahola@intel.com