drm/i915/xe2lpd: Add fake PCH

Xe2_LPD doesn't have south display engine on a PCH, it's actually
on the SoC die (while north display engine is on compute die). As
such it makes no sense to go through the PCI devices looking for
an ISA bridge. The approach used by BXT/GLK can't be used here since
leaving it with PCH_NONE would mean taking the wrong code paths.

For the places we currently use a PCH check, it's enough for now to just
check the north display version. Use that to define a fake PCH to be
used across the driver. Eventually these PCH checks may need to be
re-designed as this is already the third platform using/needing a
fake PCH.

v2: Match on display IP version rather than on platform (Matt Roper)
v3: Extend and clarify commit message (Matt Roper / Ville)

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
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/20230919192128.2045154-6-lucas.demarchi@intel.com