drm/panel: otm8009a: No error msg if probe deferred

Do not print an error message if the regulator framework returns
EPROBE_DEFER.

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1553155484-13460-1-git-send-email-yannick.fertre@st.com