backlight: Fix incr/decr with percent value of 0
This now does nothing instead of incr/decr by 10%
MFC After: 3 days
PR: 253736
Approved by: re (gjb)
(cherry picked from commit 3b005d51bd0fe4d8d19fb2df4d470b6e8baebf16)
(cherry picked from commit 9ba393f2ca0fd561c1fbf96f38eb014d7f883381)