radeonsi: get rid of get_interp_param
radeonsi: get rid of get_interp_param

Replace by a simple LLVMGetParam, since ctx->no_prolog is always false.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler