msdk: make sure child context is destroyed first
The parent context shares some resources with child context, so the child context should be destroyed first, otherwise the command below will trigger a segmentation fault
$> gst-launch-1.0 videotestsrc num-buffers=100 ! msdkh264enc !
msdkh264dec ! fakesink videotestsrc num-buffers=50 !
msdkh264enc ! msdkh264dec ! fakesink