R600: Use EXPORT_RAT_INST_STORE_DWORD for stores on Cayman

We were using RAT_INST_STORE_RAW, which seemed to work, but the docs
say this instruction doesn't exist for Cayman, so it's probably safer
to use a documented instruction instead.

Reviewed-by: Vincent Lejeune<vljn at ovi.com>
llvm-svn: 184015