gpu: ipu-di: silence videomode logspam

Adapting the videomode to the hardware constraints is something that
can and must happen during normal operation and isn't something that
the user can avoid. So printing a warning each time it happens isn't
helpful.

Demote this message to the debug level.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>