hw/vfio: Compile display.c once

hw/vfio: Compile display.c once

display.c doesn't rely on target specific definitions,
move it to system_ss[] to build it once.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20250308230917.18907-8-philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20250311085743.21724-9-philmd@linaro.org
Signed-off-by: Cédric Le Goater <clg@redhat.com>