Alpha formats in DRM are assumed to be premultiplied, so we should be
setting the PREMULT bit in the plane configuration for HVS.

Changes from v1:
 - Use correct has_alpha
Signed-off-by: Stefan Schake <stschake@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1520556817-97297-2-git-send-email-stschake@gmail.com