drm/imx: prefer snprintf over sprintf

This will trade the W=1 warning -Wformat-overflow to
-Wformat-truncation. This lets us enable -Wformat-overflow subsystem
wide.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/14c0108a54007a8360d84162a1d63cba9613b945.1704908087.git.jani.nikula@intel.com