drm/fourcc: Add a alpha field to drm_format_info

There's a bunch of drivers that duplicate the same function to know if a
particular format embeds an alpha component or not.

Let's create a field in the drm_format_info to avoid duplicating that logic
and looking up formats all the time.

Cc: Eric Anholt <eric@anholt.net>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Mark Yao <mark.yao@rock-chips.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.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/9cd9951d147ff810c1f6f68d79e7983361ed6b68.1516617243.git-series.maxime.ripard@free-electrons.com