At the end of initialization, a delay is required by the panel. Without
this delay, the panel could received a frame early & generate a crash of
panel (black screen).

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/1553155445-13407-1-git-send-email-yannick.fertre@st.com