gpu: ion: Prevent deadlock with cache flushing

Cache flushing takes the mmap semaphore to validate the
address range. This can cause a deadlock in the following
scenario with two threads in the same client:

Thread 1			Thread 2
(cache operation)		(mmap)
-----------------------------------------
lock(client_lock)
				down_write(mmap_semaphore)
down_read(mmap_semaphore)
				lock(client_lock)

Fix this by doing the check for the address range before
taking the client lock. This is independent of Ion so there
is no need to have this lock locked.

Change-Id: Ibe372cf232fbad7e031ab2d38cf3a34823f43bf9
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>