drm/i915/mtl: C20 state verification

Add state verification for C20 as we have one
for C10.

V2: Use abstractation of HW readout (Gustavo)
    Drop MPLLA/B from message for TX and CMN
    parameters (Gustavo)

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com> (v1,v2)
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231109112148.309669-1-mika.kahola@intel.com