drm/stm: dsi: Avoid printing errors for -EPROBE_DEFER

Don't print error when probe deferred error is returned.

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Tested-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210222092205.32086-2-raphael.gallais-pou@foss.st.com