drm/ttm: turn ttm_bo_device.vma_manager into a pointer

Rename the embedded struct vma_offset_manager, new name is _vma_manager.
ttm_bo_device.vma_manager changed to a pointer.

The ttm_bo_device_init() function gets an additional vma_manager
argument which allows to initialize ttm with a different vma manager.
When passing NULL the embedded _vma_manager is used.

All callers are updated to pass NULL, so the behavior doesn't change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190905070509.22407-2-kraxel@redhat.com