drm/nouveau/ttm: constify static vm_operations_struct

The only usage of nouveau_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/20210209234817.55112-4-rikard.falkeborn@gmail.com
Signed-off-by: Christian König <christian.koenig@amd.com>