drm/docs: Remove the rcar alpha from the csv file

Now that we moved the rcar-du DRM driver has been switched to the generic
alpha property, remove the former property documentation from the
deprecated CSV file.

Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/04be5e2256aa8d33d9521a68a10f0b73a24f8040.1523432341.git-series.maxime.ripard@bootlin.com