zink: do not dereference NULL pointer

zink: do not dereference NULL pointer

If first_frame_done isn't set, but fence is NULL, we end up dereferncing
that NULL-pointer.

This can happen in the case where the first submitted batch has no work,
and pfence was passed as a NULL-pointer.

While we're at it, simplify the check with the surrounding code, which
also checks for a NULL-pointer here.

Fixes: e93ca92d4ae ("zink: force explicit fence only on first frame flush")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10235>