drm/exynos: decon: Make pixel blend mode configurable

The decon hardware supports different blend modes. Add pixel blend mode
property and make it configurable, by modifying the blend equation.

Tested on TM2 with Exynos 5433 CPU, on top of linux-next-20181019.

Signed-off-by: Christoph Manszewski <c.manszewski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>