drm/ingenic-drm: drop use of legacy drm_bus_flags

Replace the legacy member with the more descriptive _DRIVE_ variant.
No functional change.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630180545.1132217-3-sam@ravnborg.org