drm/exynos: mixer: Make pixel blend mode configurable

The mixer hardware supports both premultiplied alpha and
non-premultiplied alpha. Currently premultiplied alpha is default, 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>