pan/mdg: Fix non-debug compiliation

pan/mdg: Fix non-debug compiliation

Fixes error when the assert is optimized out:

   ../src/panfrost/midgard/midgard_compile.c: In function ‘output_load_rt_addr’:
   ../src/panfrost/midgard/midgard_compile.c:1644:1: error: control reaches end of non-void function [-Werror=return-type]
    }

Closes #3270

Fixes: 7781d2c2ead ("pan/mdg: Support MRT in output load lowering")
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5895>