drm/vmwgfx: use ttm_bo_move_null() when there is nothing to move

Use ttm_bo_move_null() instead of ttm_bo_assign_mem().

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608181306.90008-1-nirmoy.das@amd.com


Signed-off-by: Christian König <christian.koenig@amd.com>