drm/rockchip: 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.

Acked-by: Sandy huang <hjc@rock-chips.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/5a217e8c93eea6f0a7f6bc5883424b47dbb6c664.1516617243.git-series.maxime.ripard@free-electrons.com