drm/vmwgfx: Return true in function vmw_fence_obj_signaled()

Fix the following coccicheck warning:

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c:518:9-10: WARNING: return of 0/1
in function 'vmw_fence_obj_signaled' with return type bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>