drm/exynos: decon5433: Fix per-plane global alpha for XRGB modes

[ Upstream commit ab337fc274a1957ff0771f19e826c736253f7c39 ]

Set per-plane global alpha to maximum value to get proper blending of
XRGB and ARGB planes. This fixes the strange order of overlapping planes.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>