msm: mdss: Add missing locks in histogram collection for collect enable

The histogram state and collect enable need a spinlock around them when
being read or written to. There were a couple places where the spinlock
wasn't held when the variables were being checked. The number of spinlock
accesses has been reduced in collect as well.

Change-Id: Ic0369667b5b2bf83a19098f876f3ab48f9dcfbda
Signed-off-by: Benet Clark <benetc@codeaurora.org>