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.

Fixes: 0cce5f28 ("drm/amdkfd: Check correct memory types for is_system variable")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Natalie Vock <natalie.vock@gmx.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 3e3fcd29)
Cc: stable@vger.kernel.org # 6.12.x

drivers/gpu/drm/amd/amdgpu/gmc_v12_0.c

@@ -528,8 +528,9 @@ static void gmc_v12_0_get_vm_pte(struct amdgpu_device *adev,
 
 	bo_adev = amdgpu_ttm_adev(bo->tbo.bdev);
 	coherent = bo->flags & AMDGPU_GEM_CREATE_COHERENT;
-	is_system = (bo->tbo.resource->mem_type == TTM_PL_TT) ||
-		(bo->tbo.resource->mem_type == AMDGPU_PL_PREEMPT);
+	is_system = bo->tbo.resource &&
+		(bo->tbo.resource->mem_type == TTM_PL_TT ||
+		 bo->tbo.resource->mem_type == AMDGPU_PL_PREEMPT);
 
 	if (bo && bo->flags & AMDGPU_GEM_CREATE_GFX12_DCC)
 		*flags |= AMDGPU_PTE_DCC;