panfrost: Fix gnu-empty-initializer error.

panfrost: Fix gnu-empty-initializer error.

../src/gallium/drivers/panfrost/pan_cmdstream.c:1553:54: error: use of GNU empty initializer extension [-Werror,-Wgnu-empty-initializer]
        union mali_attr varyings[PIPE_MAX_ATTRIBS] = { };
                                                     ^

Fixes: 836686daf36c ("panfrost: Move panfrost_emit_varying_descriptor() to pan_cmdstream.c")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4198>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4198>