drm/atmel-exynos: Use the alpha format field in drm_format_info

Now that the drm_format_info has a alpha field to tell if a format embeds
an alpha component in it, let's use it.

Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cb1bdfbb481419a17cc4f6c8a1f07930136ac13f.1516617243.git-series.maxime.ripard@free-electrons.com