nvc0: add note about where the viewport mask would go

Not piping this all the way through yet, but no better place to note
this down. This will can be used with NV_viewport_array2.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/nvc0/nvc0_program.c

@@ -80,6 +80,7 @@ nvc0_shader_output_address(unsigned sn, unsigned si)
    case TGSI_SEMANTIC_CLIPDIST:      return 0x2c0 + si * 0x10;
    case TGSI_SEMANTIC_CLIPVERTEX:    return 0x270;
    case TGSI_SEMANTIC_TEXCOORD:      return 0x300 + si * 0x10;
+   /* case TGSI_SEMANTIC_VIEWPORT_MASK: return 0x3a0; */
    case TGSI_SEMANTIC_EDGEFLAG:      return ~0;
    default:
       assert(!"invalid TGSI output semantic");