drm/exynos: mixer: also allow ARGB1555 and ARGB4444

Allow the remaining alpha formats now that blending
is properly setup.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>