gallium/util: do not perform n^2 stencil blits

We already loop n times here, no point in doing n instances as well.

Fixes: e8a40715a8b ("gallium/util: add blitter-support for stencil-fallback")
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8301>
(cherry picked from commit 96ceca33c1dd69a1feed13b0e19bfc38e6f7d979)
2 files changed