r600: clean up fragment shader input scan code

r600: clean up fragment shader input scan code

For some reason, we were iterating through the code twice (first just for
instructions needing barycentrics, then for instructions and input dcls).
Move things around slightly so this is no longer necessary.
There also was a unnedeed enabling of the fixed_pt_position_gpr - this is only
needed if the per-sample interpolation comes from an input, not from an
instruction (just move the assert where it belongs) (since the sample id to
sample from comes from a tgsi src in this case, and isn't sampleID).
Otherwise there should be no functional change.

Reviewed-by: Dave Airlie <airlied@redhat.com>