[mips] Report correct location when "erroring" about the use of $at when it's not available.

Summary: This removes the FIXMEs from test/MC/Mips/mips-noat.s.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4172

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211168 91177308-0d34-0410-b5e6-96231b3b80d8