drm/bridge/sii8620: respond to feature requests

MHL3 protocol requires device to respond to feature request from peer.
This minimal answer fulfills the requirement and allows to continue
negotiation.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-9-git-send-email-a.hajda@samsung.com