drm/amdgpu: NULL-check BO's backing store when determining GFX12 PTE flags

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu: NULL-check BO's backing store when determining GFX12 PTE flags PRT BOs may not have any backing store, so bo->tbo.resource will be NULL. Check for that before dereferencing. (cherry picked from commit 3e3fcd29b505cebed659311337ea03b7698767fc)