drm/vc4: 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: Eric Anholt <eric@anholt.net>
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/38d4d0a085634a0b8308e819c846b9173d4d93df.1516617243.git-series.maxime.ripard@free-electrons.com