i965/fs: calculate first non-payload GRF using attrib slots

i965/fs: calculate first non-payload GRF using attrib slots

When computing where the first non-payload GRF starts, we can't rely on
the number of attributes, as each attribute can be using 1 or 2 slots
depending on whether they are a dvec3/4 or other.

Instead, we need to use the number of slots used by the attributes.

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