h265parse: don't invalidate the last PPS when parsing a new SPS

This is a port of gstreamer/gst-plugins-bad!2019
to h265parse.

When a SPS is received then any previous PPS remains valid. So don't clear
the PPS flag from the parser state.

This is important because there are encoders that don't generated a PPS after
every SPS.

Part-of: <gstreamer/gst-plugins-bad!2245>
29 jobs for !2245 with backport-mr-2217-into-1.18-h265parse-pps in 31 minutes and 15 seconds (queued for 8 seconds)
merge request