adv7511: Disable generator to fix Samsung monitor @ 720p

The Samsung monitors used for the demo at Linaro Connect BKK
were particularly finniky and did not like the signal being
used.

So this patch disabled the adv7511 clock generator which
allowed it to work.

This was suggested by Xinliang Liu <xinliang.liu@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>