drm/exynos/decon5433: update shadow registers iff there are active windows

drm/exynos/decon5433: update shadow registers iff there are active windows

Improper usage of DECON_UPDATE register leads to subtle errors.
If it set in decon_commit when there are no active windows it results
in slow registry updates - all subsequent shadow registry updates takes more
than full vblank. On the other side if it is not set when there are
active windows it results in garbage on the screen after suspend/resume of
FB console.

The patch hopefully fixes it.

Change-Id: I3cbd68f9af5748f74c5a878de34b6b531aa26074
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>