drm/xe: Add drm_pagemap ops to SVM

Add support for mapping device pages to Xe SVM by attaching drm_pagemap
to a memory region, which is then linked to a GPU SVM devmem allocation.
This enables GPU SVM to derive the device page address.

v3:
 - Better commit message (Thomas)
 - New drm_pagemap.h location
v5:
 - s/xe_mem_region/xe_vram_region (Rebase)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-24-matthew.brost@intel.com