tegra: video: Don't power up the SOR twice

If U-Boot is the secondary boot loader, or has been run from itself, the
SOR may already be powered up. Powering it up again causes a hang, so
detect this situation and skip it.

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