msm: mdss: Add sysfs node for histogram notification

Add support for histogram notification to userspace
using sysfs node for mdp3 driver. Change adds a
histogram sysfs node for each framebuffer which will be
utilized to notify the clients.

Change-Id: I4203378f79bf7286eb40e76b269a471deed08478
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>