viafb: make single framebuffer multiple adapter mode work

This patch implementes this mode (viafb_SAMM_ON set and
viafb_dual_fb not set) in a useful way for CRT and DVI devices.
The same content is shown on both devices in different video modes.
The first (primary) resolution must not be bigger than the secondary
one and determines the visible region. The same content is shown
centered on the secondary output.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>