viafb: Fix various resource leaks during module_init()

The current code executed from module_init() in viafb does not have
proper error checking and [partial] resoure release paths in case
an error happens half way through driver initialization.

This patch adresses the most obvious of those issues, such as a
leftover i2c bus if module_init (and thus module load) fails.

[jc: fixed merge conflicts]
[jc: also restored -ENOMEM return on ioremap() fail]

Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Harald Welte <HaraldWelte@viatech.com>