backend/session: operate on wlr_device

Instead of operating on FDs in {open,close}_device, operate on
wlr_devices. This avoids the device lookup in wlr_session and allows
callers to have access to wlr_device fields.

For now, we use it to remove wlr_session_signal_add and replace it with
a more idiomatic wlr_session.events.change field. In the future, other
events will be added.

include/backend/drm/drm.h

@@ -82,6 +82,7 @@ struct wlr_drm_backend {
 	bool addfb2_modifiers;
 
 	int fd;
+	struct wlr_device *dev;
 
 	size_t num_crtcs;
 	struct wlr_drm_crtc *crtcs;