drm/stm: ltdc: check memory returned by devm_kzalloc()

devm_kzalloc() can fail and return NULL pointer. Check its return status.
Identified with Coccinelle (kmerr.cocci script).

Fixes: 484e72d3 ("drm/stm: ltdc: add support of ycbcr pixel formats")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230531072854.142629-1-claudiu.beznea@microchip.com


Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>