asahi: Expose VS_LAYER_VIEWPORT behind a flag

We can't technically expose the extension without a higher GL version, but the
implemented subset should work and this lets us test with piglit.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>

src/gallium/drivers/asahi/agx_pipe.c

@@ -1754,6 +1754,9 @@ agx_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_MAP_UNSYNCHRONIZED_THREAD_SAFE:
       return 1;
 
+   case PIPE_CAP_VS_LAYER_VIEWPORT:
+      return is_deqp;
+
    default:
       return u_pipe_screen_get_param_defaults(pscreen, param);
    }