Wait for the seek to finish before returning to the caller.
Lock the allocator when re-initing the DXVA2 decoder

This avoids a race condition with the allocator being decommited while the
decoder creation is still in progress.
Revert "Wait for the seek to finish before returning to the caller."

This reverts commit 5d444eac2fb31fccefa41fcf35ee3613537bc3c4.

Locking the main thread of several players while the graph is in motion
may result in bad locks, so it should be avoided.