drm/xe: Move 'vm_max_level' flag back to platform descriptor

drm/xe: Move 'vm_max_level' flag back to platform descriptor

The number of page table levels for PPGTT virtual addresses is something
that should be tracked at the platform level rather than the IP level.
Even when mixing and matching various graphics, media, and display IP
blocks, the platform as a whole has to have consistent page table
handling.  This is also a trait that should be tied to the platform even
if the graphics IP itself is not present (e.g., if we disable the
primary GT via configfs).

v2:
 - Drop default value of 4 and explicitly set the value in each platform
   descriptor.  (Lucas)
v3:
 - Drop outdated code comment and commit message paragraph about default
   value.  (Gustavo)
v4:
 - Add missing setting for tgl_desc.  (Gustavo)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20251013200944.2499947-29-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>