drm/exynos: decon: Make plane alpha configurable

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

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>