drm/xe/bo: Block mmap of DONTNEED/purged BOs

Don't allow new CPU mmaps to BOs marked DONTNEED or PURGED.
DONTNEED BOs can have their contents discarded at any time, making
CPU access undefined behavior. PURGED BOs have no backing store and
are permanently invalid.

Return -EBUSY for DONTNEED BOs (temporary purgeable state) and
-EINVAL for purged BOs (permanent, no backing store).

The mmap offset ioctl now checks the BO's purgeable state before
allowing userspace to establish a new CPU mapping. This prevents
the race where userspace gets a valid offset but the BO is purged
before actual faulting begins.

Existing mmaps (established before DONTNEED) may still work until
pages are purged, at which point CPU faults fail with SIGBUS.

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@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/20260326130843.3545241-9-arvind.yadav@intel.com

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