drm/msm/sde: support connector/crtc events

This patch adds support for internal driver events:
- direct callbacks may be registered against the sde connector,
and triggered via a new "trigger event" sde connector function;
this is suitable for simple event handlers that may be run
from within an ISR context
- delayed event handling may be posted against CRTCs, allowing
for event processing to occur within a worker thread rather
than the originating context (e.g., ISR handlers)

CRs-Fixed: 2005395
Change-Id: Idb0439892dfbda72d3c0ada882915492e3836ad8
Signed-off-by: Clarence Ip <cip@codeaurora.org>