In 913d7c15f79da33aa97100286df5d8f88731e252 stricter error checking was
introduced to the strtol call, which broke reading backlight values.
Since every sysfs backlight file ends with a newline.
As noted in a comment in the previous MR to prevent damaged pointers
after calling asprintf, replace every asprintf call with str_printf.
Previous-MR: https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/543
Signed-off-by: Sören Meier <soerenmeier@livgood.ch>