[E81] Add enable-gpio and backlight to eDP-displays

In the DP-driver there is "qcom,edp-vcc-en-gpio" that a gpio can be
assigned to in the DT and that powers on the display. But this gpio is
switched AFTER the display has been powered on. Thus there is no other
way to turn on a display but using a hog-gpio.

There is no way to assign a backlight-device to (e)DP, not even a PWM,
only a bunch of gpios. So the pwm is configured as gpio, meaning there is
no way to adjust the backlight-brightness but 2 ways to switch the
backlight off, because both gpios must be high.