msm: mdss: Fix locking error in histogram setup

Previously, during histogram setup during dspp setup, there
was a check on histogram collection enable that was not protected
by the histogram mutex and spinlock. It has been added.

Change-Id: I28770486d31da6b9b8b7420e98f0e6a5a23ed0c2
Signed-off-by: Benet Clark <benetc@codeaurora.org>