drm/exynos: fimd: Make pixel blend mode configurable

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

Tested on TRATS2 with Exynos 4412 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>