ppc/spapr/vga: Switch VGA endian on H_SET_MODE

When the guest switches the interrupt endian mode, which essentially
means a global machine endian switch, we want to change the VGA
framebuffer endian mode as well in order to be backward compatible
with existing guests who don't know about the new endian control
register.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>