[RFC] drm/exynos: rework layer blending setup

Hello,

here's the rework of the layer blending setup that I discussed with Joonyoung in the past days. There's still some TODOs in the code, but more or less it does what it's supposed to do. What still bothers me a bit is that I currently call blending reconfig in mixer_cfg_layer(). It would be nice if this could be reduced to one call per "frame" (so with the last win_{commit,disable} call). Maybe atomic provides such an infrastructure?

With best wishes, Tobias