Simplify and centralize multisample counts.

Change-Id: I012bb669444e28f844c5571ff639b31dd1a35e1d
Reviewed-on: https://swiftshader-review.googlesource.com/3950
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>