intel/fs: Split fetch_payload_reg() into separate helper for barycentrics.

intel/fs: Split fetch_payload_reg() into separate helper for barycentrics.

We're about to change the layout of barycentric vectors, which will
involve permuting the GRFs of barycentrics fetched from the thread
payload.  Make room for this in a function separate from the generic
fetch_payload_reg(), since the permutation will only be applicable to
barycentric vectors.  This allows simplifying fetch_payload_reg(),
since there was no need for handling multiple-component payload
registers except for barycentrics.

This causes some minor shader-db noise due to the new helper emitting
a LOAD_PAYLOAD instruction unconditionally, but it will be cleaned up
shortly.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>