[RFC] drm/exynos: rework layer blending setup

[Tobias Jakobi]

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