drm/bridge/sii8620: improve gen2 write burst IRQ routine

The patch adds code to report back feature complete IRQ, and code
to read and drop burst writes from peer.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-23-git-send-email-a.hajda@samsung.com