viafb: Separate global and fb-specific data

This patch moves data of interest into a new viafb_dev structure which
describes the device as a whole; the idea here is to create a separation
between what all devices may need and what the framebuffer device in
particular needs.

I've also made some small steps toward thinning out the global.h mess.

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>