viafb: make procfs entries optional

This patch adds a config option to enable procfs entries for direct
hardware access. This was the old behaviour but the option defaults
to no as this is really ugly and should not be needed if the driver
works correct (and if it doesn't, it needs to be fixed).
That stuff is really something that should
- not be needed at all (the driver should be capable of doing it)
- not be there (debugfs would be better for such things)
So add this option just for backwards compatiblity.
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>