Utility function roundUpPow2 is more optimal than roundUp so use it.

Bug: b/166462979
Change-Id: I616fa9f487b818137b1b496d93e292c3bd1f428c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2377119Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>