drm: fix oops in drm_atomic_set_crtc_for_connector

crtc can be NULL. connector, extracted from conn_state, can't.

Fixes: e3aae683

 ("drm: convert drm_atomic_uapi.c to new debug helpers")
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/7xhyNYrWtzUIt3HNrWfi9iScW0k475RZiKNfF5TbPs@cp4-web-031.plabs.ch