drm: virtio: add virtio_gpu_translate_format

Factors out code, no functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403070845.10793-2-kraxel@redhat.com