drm/i915: emit a hotplug event on resume

This will poke userspace into probing for configuration changes that may
have occurred across suspend/resume.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>