iris: Enable fragment shader interlock on Gen9+.

There's some debate about whether we should support this on older
hardware as well.  Currently i965 turns it off on Gen8- though, so
we follow suit.  If this changes, we can update this as well.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/iris/iris_screen.c

@@ -197,6 +197,7 @@ iris_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_POST_DEPTH_COVERAGE:
    case PIPE_CAP_SHADER_STENCIL_EXPORT:
    case PIPE_CAP_DEPTH_CLIP_DISABLE_SEPARATE:
+   case PIPE_CAP_FRAGMENT_SHADER_INTERLOCK:
       return devinfo->gen >= 9;
    case PIPE_CAP_MAX_DUAL_SOURCE_RENDER_TARGETS:
       return 1;