drm/bridge: analogix_dp: set the DPCD600 during disabling PSR

It looks like the BOE panel FW didn't ack the DPCD600 signal from the
host device, this will cause the panel to hang on the startup display.

The root cause is that we use the fast link mode when we enter and
exit PSR, the issue is gone if we switch from the fast link to main
link mode.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1481338159-7189-1-git-send-email-wxt@rock-chips.com