drm/exynos: mixer: Make plane alpha configurable

The mixer hardware supports variable plane alpha. Currently planes are
opaque, make this configurable.

Tested on Odroid-U3 with Exynos 4412 CPU, kernel next-20180913
using modetest.

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