drm/virtio: set fb_modifiers_not_supported

Without this, the drm core advertises LINEAR modifier which is
incorrect.

Also userspace virgl does not support modifiers.  For example, it causes
chrome on ozone/drm to fail with "Failed to create scanout buffer".

Fixes: 2af10429 ("drm: introduce fb_modifiers_not_supported flag in mode_config")
Suggested-by: Shao-Chuan Lee <shaochuan@chromium.org>
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220831190601.1295129-1-olvaffe@gmail.com


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