drm/sti: remove stih415-416 platform support

stih415 and stih416 platform are obsolete and no more supported.
Only stih407 and stih410 platform are maintained.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Peter Griffin <peter.griffin@linaro.org>