drm/i915/cx0: Update C10/C20 state calculation

drm/i915/cx0: Update C10/C20 state calculation

Update several functions in intel_cx0_phy.c to make PLL state
management more explicit.

Changes include
 * add 'const' qualifiers to intel_crtc_state parameter for
   cx0 state calculation functions
 * refactor C10/C20 PLL state calculations helpers to take
   explicit hardware state pointers instead of directly modifying
   'crtc_state->dpll_hw_state'

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/20251117104602.2363671-18-mika.kahola@intel.com