drm/ttm: inline drm_bo_setup_vm()

This helper is used only once and just wraps a call to
drm_vma_offset_add(). Remove this unneeded indirection to safe 10 lines of
code.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>