drm/msm/sde: clean up idle pc notification implementation

Video mode panel and command mode display uses different
logic for idle timeout management and power collapse signal
notification to client. It adds complexity for same feature
support in sde driver. This patch tries to unify the idle
power collapse management for video mode and command mode
displays. It reuses the existing infrastructure for this
feature and cleans up all duplicate logic.

Change-Id: If18c0bba70e191a2a98b82bb72f27a714589b0c6
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>