radeonsi: fix refcnt imbalance related to util_blitter_save_fragment_constant_buffer_...

radeonsi: fix refcnt imbalance related to util_blitter_save_fragment_constant_buffer_slot()

Indeed, util_blitter_clear() requires a call to
util_blitter_save_fragment_constant_buffer_slot(),
but most other blitter functions do not.

For instance, this issue is triggered with:
"piglit/bin/object-namespace-pollution glDrawPixels buffer -auto -fbo"
while setting GALLIUM_REFCNT_LOG=refcnt.log.

Fixes: 03bc7503d471 ("radeonsi: save the fs constant buffer to the util blitter context")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23856>