drm/ttm: remove embedded vma_offset_manager

No users left.  Drivers either setup vma_offset_manager themself
(vmwgfx) or pass the gem vma_offset_manager to ttm_bo_device_init
(all other drivers).

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/20190905070509.22407-9-kraxel@redhat.com