video: of: display_timing: remove two unsafe error messages

The error message would try to dereference the pointer that
just has been tested to be NULL. As those messages don't
really add any value without the info that the np could
provide, just remove them.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>