drm/i915/xe2lpd: Re-order DP AUX regs

The address of CTL and DATA registers for DP AUX were changed in Xe2_LPD:
now they are all in a single range, with CH_A and CH_B coming right after
the USBC instances. Like was done when moving registers to PICA, use
a helper macro to remap the ch passed to an index that can be used to
calculate the right offset.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919192128.2045154-12-lucas.demarchi@intel.com