msm: mdss: Fix locking errors in histogram disable

There is a histogram variable that was missing the spinlock while it
was being read. It has been added.

Change-Id: I5f6d4246332a84c6a4cb3503ad7bbfaddb4a0fa8
Signed-off-by: Benet Clark <benetc@codeaurora.org>
msm: mdss: Fix locking errors in histogram setup

There is a check on histogram enable in histogram setup that was not
protected by the spinlock. It has been added.

Change-Id: I769422c803bb12de2030115a08d60ad7c7f38f15
Signed-off-by: Benet Clark <benetc@codeaurora.org>