Copter: Mode init - update pos_target.z condition

Fix to avoid throttle peak on Alt_controlled modes init.
If previous mode was already Alt_controlled, pos_target.z should be
close to current altitude
In this case, don't init pos_target to current altitude but let the
alt_controller continue what he was doing (compensate the current
alt_error)
So, just call get_stopping_point_z if previous mode was stabilize or
alt_error is high (>20cm).