utils.c: fix fallback code for fence_image_create_bits()

Used a wrong variable name, causing:
/home/pq/git/pixman/demos/../test/utils.c: In function ‘fence_image_create_bits’:
/home/pq/git/pixman/demos/../test/utils.c:562:46: error: ‘width’ undeclared (first use in this function)

Use the correct variable.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Ben Avison <bavison@riscosopen.org>