drm/panel: lvds: Add support for the power-supply property

A significant number of panels need to power up a regulator in order to
operate properly. Add support for the power-supply property to enable and
disable such a regulator whenever needed.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0c0819bdf88fa948188df95e57a10820a8a4548d.1513854122.git-series.maxime.ripard@free-electrons.com