lavapipe: Fix struct initialization

lavapipe: Fix struct initialization

Fix defect reported by Coverity Scan.

Evaluation order violation (EVALUATION_ORDER)
write_write_typo: In box.x = box.x = copy->imageOffset.x, box.x is written twice with the same value.

Fixes: 9e9d90c6c38 ("lavapipe: VK_EXT_host_image_copy")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24531>