drm/sun4i: defer only if we didn't find our panel

Our code currently defers our probe on any error, even if we were not
expecting to have one at all.

Make sure we return -EPROBE_DEFER only when we were supposed to have a
panel, but it's not probed yet.

Also fix a typo while we're at it.

Fixes: 29e57fab ("drm: sun4i: Add RGB output")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>