[Backlight] Use ceiling rather than truncation in driver.

Match Fuchsia driver values to CastOS'.

Bug: b/178251082
Fixed: b/178251082
Test: Logging shows identical low brightness values
Test: fx test ti-lp8556-test
Test: fx test brightness_manager_test
Change-Id: If1219208d6c31a66c3bef90f6b9962b1fbb3d1dc
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/474819
Reviewed-by: Ernest Holmes <ernestholmes@google.com>
Reviewed-by: Adam Barth <abarth@google.com>
Commit-Queue: Kevin Twidle <kpt@google.com>
(cherry picked from commit 7d514f30322ed7149074d2db2443f9c81fd28e61)