viafb: seperate hardware initialization

This patch splits the hardware initialization out of the modesetting
function which will make it easier to make it conditional and not
require starting from scratch for every mode change, which is
especially annoying in dual framebuffer mode.
This required some minor reordering but it should not influence its
functionality.

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