drm/i915/cx0: Determine Cx0 PLL DP mode from PLL state

drm/i915/cx0: Determine Cx0 PLL DP mode from PLL state

The Cx0 PLL enable programming needs to know if the PLL is in DP or HDMI
mode. The PLL manager framework doesn't pass the CRTC state to the PLL's
enable hook, so prepare here for the conversion to use the PLL manager
for Cx0 PHY PLLs by determining the DP/HDMI mode from the PLL state.

For C10 PHYs use the fact that the HDMI divider value in the PLL
registers are set if and only if the PLL is in HDMI mode.

For C20 PHYs use the DP mode flag programmed to the VDR SERDES register,
which is set if and only if the PLL is in DP mode.

Assert that the above PLL/VDR SERDES register values match the DP/HDMI
mode being configured already during state computation.

This also allows dropping the is_dp param from the
__intel_cx0pll_enable() function, since it can retrieve this now from
the PLL 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/20251117104602.2363671-12-mika.kahola@intel.com