Fix vertex attribute count constant.

Change-Id: Ic0e4831bf3459d54b51e63ebe50b7394fb54e14c
Reviewed-on: https://swiftshader-review.googlesource.com/3562
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>