drm/stm: ltdc: check number of endpoints

Number of endpoints could exceed the fix value MAX_ENDPOINTS(2).
Instead of increase simply this value, the number of endpoint
could be read from device tree. Load sequence has been a little
rework to take care of several panel or bridge which can be
connected/disconnected or enable/disable.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1582877258-1112-1-git-send-email-yannick.fertre@st.com