panel-dt: switch sel6-8-gpio to low in unprepare

There is an issue with a display showing artifacts after leaving the display blank for some time. If sel6_8 line stays high during the power-off of the display, the display gets side-powered by the sel6_8 line. And the power sequence during the next power up does not reset the display in a good state.

So switch the sel6_8 gpio to low, when the VDD-LCD is powered of, and enable it to the correct state, just before the VDD-LCD is powered on again.

(cherry picked from commit 4f570f563b1ea85d901b8db98af6ad629c4aa1ba)