drm/xe/vm: Fix BO prefetch with CONSULT_MEM_ADVISE_PREF_LOC

In the Linux kernel, the following vulnerability has been resolved: 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 (cherry picked from commit d9a4906ac03be9f6ed3f3b45c56c866b867fd75b)