Return failure when the enum-type output property values are not found

Failure to do so causes xrandr to report incorrect property values.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

src/drmmode_display.c

@@ -1148,6 +1148,7 @@ drmmode_output_set_property(xf86OutputPtr output, Atom property,
 		    return TRUE;
 		}
 	    }
+	    return FALSE;
 	}
     }