drm/udl: Vmap framebuffer after all tests succeeded in damage handling

We now do the fast tests before the potentially expensive vmap operation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206085954.9697-4-tzimmermann@suse.de