drm/sun4i: rgb: panel is an error pointer

In case of an error, our pointer to the drm_panel structure attached to our
encoder will hold an error pointer, not a NULL pointer.

Make sure we check the right thing.

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