drm/xe/madvise: Fix ioctl argument check

drm/xe/madvise: Fix ioctl argument check

It is "preferred_mem_loc" instead of "atomic" for the ATTR_PREFERRED_LOC
path.

Also include 2 minor changes with no functional impact.
1. Remove the redundant "attr.atomic_access" assignment.
2. Replace down_read_interruptible() with
   xe_svm_notifier_lock_interruptible() to pair with
   xe_svm_notifier_unlock().

Fixes: ada7486c5668 ("drm/xe: Implement madvise ioctl for xe")
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20250911173139.1405878-2-shuicheng.lin@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>