i965/vs: Simplify fs_visitor's ATTR file.

i965/vs: Simplify fs_visitor's ATTR file.

Previously, ATTR was indexed by VERT_ATTRIB_* slots; at the end of
compilation, assign_vs_urb_setup() translated those into GRF units,
and converted ATTR to HW_REGs.

This patch moves the transslation earlier, making ATTR work in terms of
GRF units from the beginning.  assign_vs_urb_setup() simply has to add
the number of payload registers and push constants to obtain the final
hardware GRF number.  (We can't do this earlier as those values aren't
known.)

ATTR still supports reg_offset; however, it's simply added to reg.
It's not clear whether this is valuable or not.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>