configure, external ffmpeg: do not check if libavcodec contains vdpau functions.

it will fall back to other methods if not present and we have no guarantee that the libavcodec version we build against will be the same we run with (except it has the same ABI but ff_vdpau symbols are not part of the ABI).
Merge pull request #2874 from aballier/vdpau_check

configure, external ffmpeg: do not check if libavcodec contains vdpau fu...