drm/amdgpu/ttm: constify static vm_operations_struct

The only usage of amdgpu_ttm_vm_ops is to assign its address to the
vm_ops field in the vm_area_struct struct. Make it const to allow the
compiler to put it in read-only memory.
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209234817.55112-2-rikard.falkeborn@gmail.comSigned-off-by: Christian König <christian.koenig@amd.com>

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

@@ -2002,7 +2002,7 @@ static vm_fault_t amdgpu_ttm_fault(struct vm_fault *vmf)
 	return ret;
 }
 
-static struct vm_operations_struct amdgpu_ttm_vm_ops = {
+static const struct vm_operations_struct amdgpu_ttm_vm_ops = {
 	.fault = amdgpu_ttm_fault,
 	.open = ttm_bo_vm_open,
 	.close = ttm_bo_vm_close,