msm: mdss: Change histogram states atomically

For dual DSI usecases histogram has to be collected from two dspp's
for a single frame. Once driver receives the histogram done interrupt
from all the dspp's attached to logical display it will notify the
driver clients. Driver client reads the histogram and then updates the
states of histogram to signal the driver to notify next interrupt.
State tranistions of histogram should be done atomically for logical
display to ensure that data is collected from the same frame

Change-Id: Iecdb3ca4bf7be406a37d0f7b8fe61feca7b973ef
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>