Refactor sampler LOD calculation method into an enum.

Change-Id: I0beed96cd68608ce07aec0b11f14d6f61e67e53d
Reviewed-on: https://swiftshader-review.googlesource.com/5034
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>