AMDGPU: Correct getMaxNumSGPR treatment of flat_scratch

This was approximating the entry point logic for flat_scratch_init,
which is not really the point. We need to account for whether we need
to reserve the SGPR pair used for flat_scratch, not whether we needed
the initialization kernel argument. If this was an arbitrary function,
we would end up over-reporting the number of potentially free
SGPRs. The logic for architected flat scratch also only applies to the
initialization in the kernel, not the reserved registers at the end.

Avoids compile failures in a future patch from allocating more SGPRs
than the subtarget supports.