R600/SI: Fix allocating flat_scr_lo / flat_scr_hi

Only the super register flat_scr was marked as reserved,
so in some cases with high register usage it would still
try to allocate the subregisters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222737 91177308-0d34-0410-b5e6-96231b3b80d8