[C57] Rework LVDS and eDP to remove always-on regulators

The regulators related to `vcc_blk_sw` and `vcc_lcd_sw` have been
modified removing the `regulator-always-on` entry. This change requires
that:
- the backlight node has the `enable-gpio` and the `power-supply`
correctly set (now added);
- the panel node has the `backlight`, `power-supply` and `enable-gpio`
correctly set (now added).

The regulator enabling the backlight has been removed because it has the
same function of `enable-gpio` in the backlight node.

Additionally, all the display-related nodes have been refactored to
move all the related nodes into their overlays.