drm/vc4: Enable syncobj support

This doesn't require any additional functionality from the driver but
is a prerequisite to userland calling the syncobj ioctls.

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/1524607427-12876-4-git-send-email-stschake@gmail.com