drm/qxl: support 64bit surface bar

qxl devices can have a 64bit surface bar, which is quite handy if
you need a bit more surface memory.  So try to use it if it is
present.  Note that this bar might be mapped above 4g.

QEMU command line to check that out:

    qemu-system-x86_64 -m 4g \
        -vga qxl -global qxl-vga.vram64_size_mb=512 \
        $otheroptions

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>