DisplayModeControl: Add null check to setMode()

* getCurrentMode() can return null. Don't let setMode() try set a
  null mode because bad things happen.
* Let's make cmhw safe again (groan, I know that one was bad)

Change-Id: I20bdee4a0e8cbb0598f35770d71902332db51ddb

cmhw/org/cyanogenmod/hardware/DisplayModeControl.java

@@ -113,6 +113,10 @@ public class DisplayModeControl {
      * if this mode is valid.
      */
     public static boolean setMode(DisplayMode mode, boolean makeDefault) {
+        if (mode == null) {
+            return false;
+        }
+
         boolean success = FileUtils.writeLine(MODE_PATH, String.valueOf(mode.id));
         if (success && makeDefault) {
             return FileUtils.writeLine(DEFAULT_PATH, String.valueOf(mode.id));