GStreamer: Prevent calling CameraBinSession::setStateHelper twice

Since CameraBinSession::setStateHelper() is supposed to handle
only pending states, added a fix to prevent calling it twice.
Otherwise CameraBinSession::load() can be called few times
which might lead an error from gstreamer.

Possible call stack:
CameraBinSession::setState()
CameraBinSession::setStateHelper()
CameraBinSession::load()
CameraBinSession::setStatus()
CameraBinSession::setStateHelper()
CameraBinSession::load() << gst_element_set_state is called also twice

Task-number: QTBUG-53204
Change-Id: I00c66f91cd3b885c70848245da725ff68943fad2
Reviewed-by: Christian Stromme <christian.stromme@qt.io>