gallium/util: set right dst-dimensions

This should have been the destination surface size, not the dimensions
of the source box. These were the same in the test-case I used while
developing this, but this matters for the GTF framebuffer-blit
functional CTS tests.

Fixes: e8a40715a8b ("gallium/util: add blitter-support for stencil-fallback")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6898>
1 file changed