r300,r600,svga: save the FS constant buffer for u_blitter to fix a regression

r300,r600,svga: save the FS constant buffer for u_blitter to fix a regression

Fixes: 773a23eb6da - gallium/u_blitter: clear color buffers using color from a constant buffer
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6548
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6539

Tested-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Tested-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16711>