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

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

The number of virtual address bits 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 the default value of 48 and explicitly set it in each relevant
   descriptor.  (Lucas, Michal)
v3:
 - Drop an outdated comment about default value.  (Michal)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20251013200944.2499947-28-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>