drm/stm: ltdc: fix deferred endpoint management

When a driver related to one of the endpoints is deferred
due to probe dependencies (i2c, spi...) but the other one
is ready, ltdc probe continues and the deferred driver
will never be probed again.

The fix consists in waiting for all deferred endpoints before
continuing the ltdc probe.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180417113441.8214-1-philippe.cornu@st.com