drm/i915/cx0: Read out power-down state of both PHY lanes for reversed lanes

drm/i915/cx0: Read out power-down state of both PHY lanes for reversed lanes

For a port used with lane reversal enabled the first two TX lanes will
be enabled in PHY lane#1 instead of PHY lane#0. At the moment the HW
readout will read out the power-down state for these two TX lanes from
PHY lane#0 incorrectly. The display HW lane reversal feature (vs. the
similar TCSS lane swap) is only used for TypeC legacy mode and for
non-TypeC PHYs. Since in both of these cases the display owns both PHY
lanes, both of these PHY lanes' state can be read out. Do that to fix
cases when lane reversal is used with 1 or 2 active TX lanes.

While at it add an assert to the PLL enable function about the above
assumption on when lane reversal can be used.

Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Fixes: 230d4c748113 ("drm/i915/cx0: Track the Cx0 PHY enabled lane count in the PLL state")
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20251120172358.1282765-5-imre.deak@intel.com