compositor-drm: Delete drm_backend_set_modes

Even if we do have a framebuffer matching the mode, we immediately
schedule a repaint, meaning we either do work for no reason, or show
stale content before we bring up the new content.

Delete this and just let repaint deal with it.

Differential Revision: https://phabricator.freedesktop.org/D1481



Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>