drm/ingenic: Handle errors of drm_atomic_get_plane_state

drm_atomic_get_plane_state() can return errors, so we need to handle
these.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728151641.26124-2-paul@crapouillou.net