mpegtsmux: restore stream creation order

mpegtsmux: restore stream creation order

In 7c767f3fcd5a7b40d205bb4d588dad6c6275c729 , 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.