drm/xe: Stub out new pagefault layer

drm/xe: Stub out new pagefault layer

Stub out the new page fault layer and add kernel documentation. This is
intended as a replacement for the GT page fault layer, enabling multiple
producers to hook into a shared page fault consumer interface.

v2:
 - Fix kernel doc typo (checkpatch)
 - Remove comment around GT (Stuart)
 - Add explaination around reclaim (Francois)
 - Add comment around u8 vs enum (Francois)
 - Include engine instance (Stuart)
v3:
 - Fix XE_PAGEFAULT_TYPE_ATOMIC_ACCESS_VIOLATION kernel doc (Stuart)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Tested-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patch.msgid.link/20251031165416.2871503-2-matthew.brost@intel.com