drm: omapdrm: dpi: Don't treat GPIO probe deferral as an error

There's no need to print an error message on probe deferral, that's a
normal situation. Probe deferral debugging can be performed by enabling
the related debug messages in the drivers core.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>