gpu: ion: Don't recursively take secure chunk lock

During allocation, the chunk_lock is taken to protect the chunk
list. dma_alloc_coherent may invoke the shrinker which also tries
to take the chunk_lock. This leads to a deadlock. If the mutex is
already locked in the shrinker, just return -1 as specifed by the
shrinker API to signal this condition.

Change-Id: I5525ecfcdc6deea61024f0008c62026103728571
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>