drm/bridge/sii8620: split EDID read and write code

MHL3 requires that after reading EDID from the sink source should ask
peer for features. To make both protocols happy the patch splits the code
accordingly.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1485935272-17337-18-git-send-email-a.hajda@samsung.com