viafb: Move core stuff into via-core.c

The first step toward turning viafb into a multifunction driver.  This
patch creates a new via-core.c file which serves as the main PCI driver;
everything else comes below that.  Some work has been done to rationalize
the i2c drivers in this new scheme.

Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Cc: Harald Welte <laforge@gnumonks.org>
Acked-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>