Fix using floating-point exp2()

Change-Id: I2dc3942032458093bb132d7b86c34f05e1bb8137
Reviewed-on: https://swiftshader-review.googlesource.com/4443
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>