d3d11memory: Remove GstPoll from GstD3D11PoolAllocator
GstBufferPool implementation was referenced for this GstD3D11PoolAllocator, for example GstAtomicQueue, various atomic operations, and GstPoll ones. However, such combination seems to be almost pointless since gst_poll_{read,write}_control() takes mutex and also GstPoll uses Win32 event handle internally. Use simple SRWLOCK and CONDITION_VARIABLE instead, and don't make things complicated/inefficient.