BufferQueue: alloc without holding the lock

This change makes BufferQueue::dequeueBuffer release its mutex before
allocating new buffers.  This should alleviate lock contention in
SurfaceFlinger where SF's main thread can get blocked waiting for an allocation
operation to complete.

Bug: 7335075
Change-Id: I1b000539cc616a695afab2e9c68507db69e57b13