drm/sti: Use correct printk format specifiers for size_t

The correct format specifier for size_t is %zu. Using %d (or %u)
generates a warning on 64-bit platforms. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Philippe Cornu <philippe.cornu@foss.st.com>