drm/xe/madvise: Enable purgeable buffer object IOCTL support

drm/xe/madvise: Enable purgeable buffer object IOCTL support

Hook the madvise_purgeable() handler into the madvise IOCTL now that all
supporting infrastructure is complete:

 - Core purge implementation (patch 3)
 - BO state tracking and helpers (patches 1-2)
 - Per-VMA purgeable state tracking (patch 6)
 - Shrinker integration for memory reclamation (patch 10)

This final patch enables userspace to use the
DRM_XE_VMA_ATTR_PURGEABLE_STATE madvise type to mark buffers as
WILLNEED/DONTNEED and receive the retained status indicating whether
buffers were purged.

The feature was kept disabled in earlier patches to maintain
bisectability and ensure all components are in place before exposing to
userspace.

Userspace can detect kernel support for purgeable BOs by checking the
DRM_XE_QUERY_CONFIG_FLAG_HAS_PURGING_SUPPORT flag in the query_config
response.

Suggested-by: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260326130843.3545241-12-arvind.yadav@intel.com