drm/vmwgfx: remove unused vmw_bo_is_vmw_bo() declaration

vmw_bo_is_vmw_bo() has been removed since
commit 298799a2

 ("drm/vmwgfx: Fix gem refcounting and
memory evictions"), so remove it.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913024847.552254-2-cuigaosheng1@huawei.com