lcdif: fix bridge shutdown

In dual mode (with lcdif and hdmi both used) Linux would end up
with built-in display flashing randomly at the top and otherwise
black screen. Instead of disabling the clocks, shut down the bridge
(SN65DSI86) exactly by spec and leave the clocks be.

Also increase the delay on init to make sure the bridge is up as
sometimes it did not respond in time, causing the display not work
in U-Boot.