gpu: ion: add locking to traversal of volatile rb tree

In ion_debug_heap_show we're iterating over an rb tree (dev->clients)
that could change while we're iterating. Fix this by taking the lock
that is used to control access to this tree.

CRs-Fixed: 571918
Change-Id: I6832e1e98e2d2a69fc653451d3752d43ec3ef269
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>