mdss: mdp: update dspp histogram states atomically

State of dspp histogram blocks attached to logical display
should be changed atomically to idle when histogram collect is ioctl
is called by userspace clients. This will ensure that histogram
interrupt handler will see consistent states for all dspp's attached to
logical display and prevent incorrect sysfs notification to userspace
modules.

Change-Id: I7f32ce21cd65026e1ea01e3b6fe8b571c7b08db3
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>