ion: don't wakeup refill threads for order-0 requests

We check the pool->count against the ION_POOL_LOW_MARK before deciding
to wakeup the refill threads. But as we never refill the order-0 pool,
we will keep waking up these refill threads only they goto sleep again
without doing any work. The overhead of waking up of these threads
is quite high. Fix it.

Change-Id: I0d10b96a46de24b45822ebeda1f20b75e43c2a5d
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>