For high/main profile, user can choose to use cavlc by specify "-coder cavlc",
for default, it'll will use cabac, if it's baseline, we'll use cavlc by specs anyway.

ffmpeg -y -f lavfi -i testsrc -c:v libopenh264 -profile:v main -coder cavlc -frames:v 1 -bsf trace_headers -f null -
before the patch:
entropy_coding_mode_flag                                    0 = 1

after the patch:
entropy_coding_mode_flag                                    0 = 0
Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>