drm/stm: ltdc: manage the get_irq probe defer case

Manage the -EPROBE_DEFER error case for the ltdc IRQ.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1556114601-30936-2-git-send-email-fabien.dessenne@st.com