msm: mdss: Fix locking error in histogram enable

There was a missing spinlock around the histogram enable which is
necessary. It has been added.

Change-Id: I0c06406682234134260281870b755ed559d34b9e
Signed-off-by: Benet Clark <benetc@codeaurora.org>
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 values were being checked. The number of spinlock
accesses has been reduced in collection as well.

Change-Id: I2f7c200c436ec5e062ba11a8208f88af89b25e2d
Signed-off-by: Benet Clark <benetc@codeaurora.org>