COPTER: Fix MOUNT_CONTROL yaw in guided

If the mount instance does not support yaw/pan, the copter needs to yaw in
response to MSG_MOUNT_CONTROL and MAV_COMMAND_DO_MOUNT_CONTROL commands from
a GCS or co-computer. There was no checking for mount pan in the GCS_Mavlink
logic.  As such, no yaw takes place when a mount control command calls for it.
This patch impliments copter yaw control for both MSG_MOUNT_CONTROL and
MAV_COMMAND_DO_MOUNT_CONTROL in copter GCS_Mavlink.

ArduCopter/GCS_Mavlink.cpp

@@ -1016,6 +1016,9 @@ void GCS_MAVLINK_Copter::handleMessage(mavlink_message_t* msg)
 
         case MAV_CMD_DO_MOUNT_CONTROL:
 #if MOUNT == ENABLED
+            if(!copter.camera_mount.has_pan_control()) {
+                copter.set_auto_yaw_look_at_heading((float)packet.param3 / 100.0f,0.0f,0,0);
+            }
             copter.camera_mount.control(packet.param1, packet.param2, packet.param3, (MAV_MOUNT_MODE) packet.param7);
             result = MAV_RESULT_ACCEPTED;
 #endif
@@ -1655,6 +1658,9 @@ void GCS_MAVLINK_Copter::handleMessage(mavlink_message_t* msg)
         break;
     //deprecated. Use MAV_CMD_DO_MOUNT_CONTROL
     case MAVLINK_MSG_ID_MOUNT_CONTROL:
+        if(!copter.camera_mount.has_pan_control()) {
+            copter.set_auto_yaw_look_at_heading(mavlink_msg_mount_control_get_input_c(msg)/100.0f, 0.0f, 0, 0);
+        }
         copter.camera_mount.control_msg(msg);
         break;
 #endif // MOUNT == ENABLED