gallium/aux: add PIPE_CAP_MAX_VARYINGS to u_screen

Allows drivers using `u_pipe_screen_get_param_defaults` to use a
fallback value for the new pipe cap. Default value of 8 based on GL 2.1
MAX_VARYING_FLOATS

Reviewed-by: Eric Anholt <eric@anholt.net>

src/gallium/auxiliary/util/u_screen.c

@@ -337,6 +337,9 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_DEST_SURFACE_SRGB_CONTROL:
       return 1;
 
+   case PIPE_CAP_MAX_VARYINGS:
+      return 8;
+
    default:
       unreachable("bad PIPE_CAP_*");
    }