drm/exynos: check a pixel format to a particular window layer

This patch checks if a requested window supports alpha channel or not.

In case of s3c64xx, window 0 doesn't support alpha channel so if
the request pixel format is ARGB8888 then change it to XRGB8888.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>