drm: virtio: fix virtio_gpu_mode_dumb_create

Lookup format using virtio_gpu_translate_format()
instead of hardcoding it.  Fixes xorg display on
bigendian guests (i.e. ppc64).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403070845.10793-3-kraxel@redhat.com