drm/xe/vm: Validate userptr during gpu vma prefetching

If a userptr vma subject to prefetching was already invalidated
or invalidated during the prefetch operation, the operation would
repeatedly return -EAGAIN which would typically cause an infinite
loop.

Validate the userptr to ensure this doesn't happen.

v2:
- Don't fallthrough from UNMAP to PREFETCH (Matthew Brost)

Fixes: 5bd24e78 ("drm/xe/vm: Subclass userptr vmas")
Fixes: 617eebb9 ("drm/xe: Fix array of binds")
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger.kernel.org> # v6.9+
Suggested-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/20250228073058.59510-2-thomas.hellstrom@linux.intel.com