uboot-imx

video: Allow simple-panel to be used without regulators

At present simple-panel requires regulator support and will not build
without it. But some panels do not have a power supply, or at least not one
that can be controlled. Update the implementation to cope with this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>