panfrost: Do no use designated initializer for union

panfrost: Do no use designated initializer for union

../src/panfrost/lib/tests/test-earlyzs.cpp: In function 'void test(pan_earlyzs, pan_earlyzs, uint32_t)':
../src/panfrost/lib/tests/test-earlyzs.cpp:59:4: error: 'pan_shader_info::<unnamed union>' has no non-static data member named 'can_discard'
   59 |    };
      |    ^

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18024>