drm/xe/userptr: replace xe_hmm with gpusvm

drm/xe/userptr: replace xe_hmm with gpusvm

Goal here is cut over to gpusvm and remove xe_hmm, relying instead on
common code. The core facilities we need are get_pages(), unmap_pages()
and free_pages() for a given useptr range, plus a vm level notifier
lock, which is now provided by gpusvm.

v2:
  - Reuse the same SVM vm struct we use for full SVM, that way we can
    use the same lock (Matt B & Himal)
v3:
  - Re-use svm_init/fini for userptr.
v4:
  - Allow building xe without userptr if we are missing DRM_GPUSVM
    config. (Matt B)
  - Always make .read_only match xe_vma_read_only() for the ctx. (Dafna)
v5:
  - Fix missing conversion with CONFIG_DRM_XE_USERPTR_INVAL_INJECT
v6:
  - Convert the new user in xe_vm_madise.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Dafna Hirschfeld <dafna.hirschfeld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250828142430.615826-17-matthew.auld@intel.com