lavc/avcodec: fix global/private option precendence

lavc/avcodec: fix global/private option precendence

Broken after 7753a9d62725d5bd8313e2d249acbe1c8af79ab1. Apply only the
whitelist early, and the rest with a single call to av_opt_set_dict2()
with AV_OPT_SEARCH_CHILDREN, which should be equivalent to the original
behaviour.

Reported-by: Cameron Gutman <aicommander@gmail.com>
(cherry picked from commit 9ce63e65d65b303813d4ae677228226d7cd232b9)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
Gbp-Pq: Name 0013-lavc-avcodec-fix-global-private-option-precendence.patch