nir/spirv: do not require a format with images that are not sampled
nir/spirv: do not require a format with images that are not sampled

As soon as we support shaderStorageImageWriteWithoutFormat we can see
write-only images (sampled == 2) that don't have a format specified.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler