radeonsi/nir: fix fs output index

radeonsi/nir: fix fs output index

Fixes the following piglit tests:

arb_blend_func_extended-fbo-extended-blend
arb_blend_func_extended-fbo-extended-blend-explicit
arb_blend_func_extended-fbo-extended-blend-explicit_gles3
arb_blend_func_extended-fbo-extended-blend-pattern
arb_blend_func_extended-fbo-extended-blend-pattern_gles2
arb_blend_func_extended-fbo-extended-blend-pattern_gles3
arb_blend_func_extended-fbo-extended-blend_gles3
ext_framebuffer_multisample/alpha-to-coverage-dual-src-blend
ext_framebuffer_multisample/alpha-to-one-dual-src-blend

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>