[Graphite] Bin Uniform Buffer binding sizes into powers of 2.

The goal here is to reduce the number of uniform descriptor sets that
we need to create which are keyed on the size of the binding.

Change-Id: I04768e97a446571f6d48b31ce8e71ca1b802bbd1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1203836
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>