drm/msm/sde: add support for histogram event

Add histogram event support to notify histogram data done.
Read histogram data in event notification function and store
the data into the blob. Notify userspace with the blob id.

Change-Id: If74457e66c5f804c99d2667f866779656d349fbe
Signed-off-by: Xu Yang <yangxu@codeaurora.org>