viafb: rework the I2C support in the VIA framebuffer driver

This patch changes the way how the various I2C busses are used internally
inside the viafb driver:  Previosuly, only a single i2c_adapter was created,
even though two different hardware I2C busses are accessed: A structure member
in a global variable was modified to indicate the bus to be used.

Now, all existing hardware busses are registered with the i2c core, and the
viafb_i2c_{read,write}byte[s]() function take the adapter number as function
call parameter, rather than referring to the global structure member.

[jc: even more painful merge with mainline changes ->2.6.34]
[jc: painful merge with OLPC changes]

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>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>