rockchip: video: Lower hpd wait time

Waiting 30 seconds for the hpd to go high seems a bit much, especially
on headless boots. Lowering the timeout to 300ms.

Sending as RFC because frankly i don't know what a sensible timeout is
here, but 30 seconds is clearly not it :)

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Dropped RFC tag:
Signed-off-by: Simon Glass <sjg@chromium.org>