drm/radeon: fix VM IB handling

Calling radeon_vm_bo_find on the IB BO during CS
is illegal and can lead to an crash.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_vm.c b/drivers/gpu/drm/radeon/radeon_vm.c
index 2726b46..fa41e0d 100644
--- a/drivers/gpu/drm/radeon/radeon_vm.c
+++ b/drivers/gpu/drm/radeon/radeon_vm.c
@@ -1007,6 +1007,7 @@
 	int r;
 
 	vm->id = 0;
+	vm->ib_bo_va = NULL;
 	vm->fence = NULL;
 	vm->last_flush = NULL;
 	vm->last_id_use = NULL;