drm/exynos: mixer: refactor layer setup

Properly configure blending properties of given hardware layer based on
the selected pixel format. Currently only per-pixel-based alpha is possible
when respective pixel format has been selected. Configuration of global,
per-plane alpha value, color key and background color will be added later.

This patch is heavily inspired by earlier work done by Tobias Jakobi
<tjakobi@math.uni-bielefeld.de>.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>