display/g364fb: Convert sysbus init function to realize function

display/g364fb: Convert sysbus init function to realize function

Use DeviceClass rather than SysBusDeviceClass in
g364fb_sysbus_class_init().

Cc: pbonzini@redhat.com
Cc: kraxel@redhat.com
Cc: f4bug@amsat.org
Cc: alistair.francis@wdc.com
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181130093852.20739-6-maozhongyi@cmss.chinamobile.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>