drm/panel: lvds: Handle the optional regulator case properly

The devm_regulator_get_optional function, unlike it was assumed in the
commit a1c55bcc

 ("drm/panel: lvds: Add support for the power-supply
property"), is actually returning an error pointer with -ENODEV instead of
NULL when there's no regulator to find.

Make sure we handle that case properly.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: a1c55bcc

 ("drm/panel: lvds: Add support for the power-supply property")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110155941.16109-1-maxime.ripard@free-electrons.com