drm/bridge/sii8620: fix HDMI cable connection to dongle

MHL bridge is usually connected to TV via MHL dongle. Currently plugging
HDMI cable to dongle is handled improperly.

Fix it by splitting connecting of a dongle and a HDMI cable. The driver
should now handle unplugging a sink from a dongle and plugging a
different sink with new edid.

Tested on MHL1, MHL2 and MHL3 using various vendors' dongles both in
DVI and HDMI mode.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1516705996-8928-1-git-send-email-m.purski@samsung.com