Fixed mask related state queries

Mask queries could return negative values when only positive
values are accepted, so a clamp was added to prevent returning
negative values.

Change-Id: Ib151573fff54604567b83736163b8fe748c749f8
Reviewed-on: https://swiftshader-review.googlesource.com/4743
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>