util/format: Change the pointer offset.

Changed the pointer offset to 2 to account for the second structure variable.

Fixes: 90f98b56 ("mesa: Deduplicate _mesa_pack_uint_z_row().")
Closes: mesa/mesa#4685
Signed-off-by: Sergii Melikhov <sergii.v.melikhov@globallogic.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <mesa/mesa!11060>