viafb: try to map less memory in case of failure

The current code might result in trying to remap 512MB video ram on
a 32 bit system which is quite likely to fail.
This patch tries to map less of it down to 8MB as this should still
be enough to get a reasonably well working framebuffer. This should
make viafb work for many people without requiring them to manually
allocate more space.

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