drm/amdgpu: fixx NULL pointer deref in gmc_v9_0_get_vm_pte

We not only need to make sure that we have a BO, but also that the BO
has some backing store.

Fixes: d1a372af ("drm/amdgpu: Set MTYPE in PTE based on BO flags")
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c

@@ -1185,6 +1185,8 @@ static void gmc_v9_0_get_vm_pte(struct amdgpu_device *adev,
 				struct amdgpu_bo_va_mapping *mapping,
 				uint64_t *flags)
 {
+	struct amdgpu_bo *bo = mapping->bo_va->base.bo;
+
 	*flags &= ~AMDGPU_PTE_EXECUTABLE;
 	*flags |= mapping->flags & AMDGPU_PTE_EXECUTABLE;
 
@@ -1196,7 +1198,7 @@ static void gmc_v9_0_get_vm_pte(struct amdgpu_device *adev,
 		*flags &= ~AMDGPU_PTE_VALID;
 	}
 
-	if (mapping->bo_va->base.bo)
+	if (bo && bo->tbo.resource)
 		gmc_v9_0_get_coherence_flags(adev, mapping->bo_va->base.bo,
 					     mapping, flags);
 }