viafb: introduce per output device power management

This patch moves common parts of dvi.c, lcd.c and vt1636.c to hw.c to
start a per output device power management. There should be no runtime
changes aside that this patch enables the proc interface to enable/disable
devices when needed which greatly increases the chances that changes to
the output device configuration will work. However the power management is
not yet complete so it might fail on some configurations. As this area is
quite complex and touches undocumented things there is a slight chance of
regressions.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>