drm/i915/cx0: Read out the Cx0 PHY SSC enabled state

drm/i915/cx0: Read out the Cx0 PHY SSC enabled state

Read out the C10, C20 PHY PLLs SSC enabled state, so the PLL HW/SW state
verification can check this state as well.

C10 PHY PLLs program some PLL registers zeroed out for the non-SSC case,
while programming non-zero values to the same registers for the SSC
case, so check that these PLL registers being zero or non-zero matches
the PLL's overall SSC-enabled state (stored in the
intel_c10pll_state::ssc_enabled flag).

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/20251117104602.2363671-11-mika.kahola@intel.com