mpegtsmux: restore stream creation order
In 7c767f3f , stream creation was refactored to occur before potential program creation. This created issues with pipelines such as:
gst-launch-1.0 videotestsrc ! video/x-raw, format=I420, width=640, height=640, framerate=25/1 !
x264enc ! hlssink2 target-duration=1
eg.: gst_buffer_copy_into: assertion 'bufsize >= offset + size' failed
As this reordering was actually not needed for the purpose of allowing to specify a PCR stream, this reverts the reordering part of the initial commit.