Genlock: allow synchronization with a single gralloc handle
In order to support synchronization in a process with a single
gralloc handle we require the ability to write lock a buffer
while it is already read locked by the same handle. This change
extends the concept of an exclusive write lock or recursive read
locks to a genlock handle (rather than the genlock lock).
Genlock cannot provide deadlock protection because the same
handle can be used simultaneously by a producer and consumer.
In practice an error will still be generated when the timeout
expires.