drm/ttm: remove ttm_bo_unmap_virtual_locked declaration v2

That was missed during the cleanup.

v2: fix comment in vmwgfx as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/394092/