AP_Mount: mount_open servo output should work for all backends

[rmackay9]

The SRV_Channel library has a mount_open and mount2_open functions but they are only used by the Mount_Servo backend.

We should move the little bit of code that controls to servo into a function in AP_Mount_Backend and then call it from every backend as part of the update() call.

This issue was raised here in the Plane forums.