drm/stm: ltdc: add number of interrupts

The number of interrupts depends on the ltdc version.
Don't try to get interrupt which not exist, avoiding
kernel warning messages.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1579601632-7001-1-git-send-email-yannick.fertre@st.com