compositor: Change notify_* function to take a weston_seat

Fewer indirections and derefs, and it's also more appropriate for a
backend calling into weston core to pass a weston_seat.
diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index 933a273..8c8c8c0 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -2049,7 +2049,7 @@
 		evdev_device_destroy(device);
 
 	if (seat->base.seat.keyboard)
-		notify_keyboard_focus_out(&seat->base.seat);
+		notify_keyboard_focus_out(&seat->base);
 }
 
 static void