ppc: Include vga cirrus card into the compiling process

Drivers for this card exists on PPC-based AmigaOS guests so it is useful to
allow users to emulate the graphics card for PPC machines.

As cirrus vga is currently preferred over std(vga) in absence of any user
choice, this change also sets the default display of spapr machines to
std as otherwise qemu refuses to start these machines. Not specifying an
explicit graphics mode is for instance done by 'make check'.

Signed-off-by: Sebastian Bauer <mail@sebastianbauer.info>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>