drm/gpusvm, drm/pagemap: Move migration functionality to drm_pagemap

drm/gpusvm, drm/pagemap: Move migration functionality to drm_pagemap

The migration functionality and track-keeping of per-pagemap VRAM
mapped to the CPU mm is not per GPU_vm, but rather per pagemap.
This is also reflected by the functions not needing the drm_gpusvm
structures. So move to drm_pagemap.

With this, drm_gpusvm shouldn't really access the page zone-device-data
since its meaning is internal to drm_pagemap. Currently it's used to
reject mapping ranges backed by multiple drm_pagemap allocations.
For now, make the zone-device-data a void pointer.

Alter the interface of drm_gpusvm_migrate_to_devmem() to ensure we don't
pass a gpusvm pointer.

Rename CONFIG_DRM_XE_DEVMEM_MIRROR to CONFIG_DRM_XE_PAGEMAP.

Matt is listed as author of this commit since he wrote most of the code,
and it makes sense to retain his git authorship.
Thomas mostly moved the code around.

v3:
- Kerneldoc fixes (CI)
- Don't update documentation about how the drm_pagemap
  migration should be interpreted until upcoming
  patches where the functionality is implemented.
  (Matt Brost)
v4:
- More kerneldoc fixes around timeslice_ms
  (Himal Ghimiray, Matt Brost)
v6:
- Fix an uninitialized pagemap pointer (CI)

Co-developed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250619134035.170086-2-thomas.hellstrom@linux.intel.com