drm/msm: add support for custom event notification

SDE hardware supports custom events which needs to be reported by driver
to user-space clients who request for notification. DRM framework
provides poll/read based interface to report the events to the
user-space clients. Change adds support to report msm driver events to
user-space clients.

Change-Id: I4ce75488ad1d1436cafb3451ebc7808961c6cab5
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>