drm/xe/vm: Fix BO prefetch with CONSULT_MEM_ADVISE_PREF_LOC

drm/xe/vm: Fix BO prefetch with CONSULT_MEM_ADVISE_PREF_LOC

When prefetch region is DRM_XE_CONSULT_MEM_ADVISE_PREF_LOC for a BO VMA,
the code used it as an index into region_to_mem_type[], causing an
out-of-bounds access since the value is -1.

Resolve the preferred location for BO VMAs directly: local VRAM on dGFX
(using the BO's tile placement) or system memory on iGPU.

Discovered using AI-assisted static analysis confirmed by Intel Product
Security.

v2:
-Fix null dereference

Reported-by: Martin Hodo <martin.hodo@intel.com>
Fixes: c1bb69a2e8e2 ("drm/xe/svm: Consult madvise preferred location in prefetch")
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20260624174943.2808767-2-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
(cherry picked from commit d9a4906ac03be9f6ed3f3b45c56c866b867fd75b)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>