drm/vmwgfx: Fix possible usage of an uninitialized variable

vmw_user_bo_lookup can fail to lookup user buffers, especially because
the buffer handles come from the userspace. The return value has
to be checked before the buffers are put back.

This was spotted by Dan's Smatch statick checker:
    drivers/gpu/drm/vmwgfx/vmwgfx_bo.c:574 vmw_user_bo_synccpu_release()
	error: uninitialized symbol 'vmw_bo'.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 8afa13a0 ("drm/vmwgfx: Implement DRIVER_GEM")
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215200224.3693345-1-zack@kde.org