r600: fix refcnt imbalance related to evergreen_set_shader_images()
Indeed, the reference was overwritten.
For instance, this issue is triggered with: "piglit/bin/shader_runner tests/spec/arb_shader_image_load_store/execution/write-to-rendered-image.shader_test -auto -fbo" while setting GALLIUM_REFCNT_LOG=refcnt.log.
Fixes: a6b37928 ("r600: add core pieces of image support.") Signed-off-by: Patrick Lerda patrick9876@free.fr