fbdev: via: Fix error in via_core_init()

via_core_init() won't exit the driver when pci_register_driver() failed.
Exit the viafb-i2c and the viafb-gpio in failed path to prevent error.

VIA Graphics Integration Chipset framebuffer 2.4 initializing
Error: Driver 'viafb-i2c' is already registered, aborting...
Error: Driver 'viafb-gpio' is already registered, aborting...

Fixes: 7582eb9b

 ("viafb: Turn GPIO and i2c into proper platform devices")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>