avformat/mpegtsenc: make first_pcr sync with the first valid dts

avformat/mpegtsenc: make first_pcr sync with the first valid dts

now first_pts assume dts will start from zero, if it's not true(copyts is enable),
too many null packet will be inserted for cbr output.

Please test with below command, you'll get huge test.ts without the patch:
./ffmpeg -y -copyts -i ../fate-suite/mpegts/loewe.ts  -c:v libx264 -x264opts \
   nal-hrd=cbr:force-cfr=1 -b:v 3500k -minrate 3500k -maxrate 3500k -bufsize \
   1000k  -c:a mp2 -muxrate 4500k  -vframes 1000 test.ts

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>