Revert "libavcodec/utils.c: simplify avcodec locking with atomics"

This reverts commit 590136e7.

Atomics are not required for this variable, because it is protected
through the lock manager, and the use of atomics here is not compatible
with the c11 emulation wrappersi.

Fixes FATE on MSVC, among other setups which use the compat wrappers.