iris: Set nir_shader_compiler_options::unify_interfaces.

This is technically enabling the option in the common intel backend
code, but only the st/nir linker uses the option, so it's iris-only.

Fixes Piglit's spec/glsl-1.50/execution/geometry/clip-distance-vs-gs-out

Closes: #2274
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3249>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3249>

src/intel/compiler/brw_compiler.c

@@ -194,6 +194,9 @@ brw_compiler_create(void *mem_ctx, const struct gen_device_info *devinfo)
 
       nir_options->lower_int64_options = int64_options;
       nir_options->lower_doubles_options = fp64_options;
+
+      nir_options->unify_interfaces = i < MESA_SHADER_FRAGMENT;
+
       compiler->glsl_compiler_options[i].NirOptions = nir_options;
 
       compiler->glsl_compiler_options[i].ClampBlockIndicesToArrayBounds = true;