r600: fix refcnt imbalance related to shader

r600: fix refcnt imbalance related to shader

Indeed, gs_copy_shader was not freed.

Fixes: commit 1371d65a7fbd695d3516861fe733685569d890d0
    r600g: initial support for geometry shaders on evergreen (v2)

For instance, with "piglit/bin/shader_runner generated_tests/spec/arb_gpu_shader_int64/execution/built-in-functions/gs-abs-i64vec2.shader_test -auto -fbo"
while setting GALLIUM_REFCNT_LOG=refcnt.log.

Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22087>