i965: Update GS state for Broadwell.

i965: Update GS state for Broadwell.

This is quite similar to the Gen7 code.  The main changes:
 - 48-bit relocations
 - Thread count is specified as U/2-1 instead of U-1.
 - An extra DWord (DW9) with clip planes, URB entry output length/offsets
 - We need to program the "Expected Vertex Count" (VerticesIn)

v2: Set the number of binding table entries so they can be prefetched
    (requested by Eric Anholt).
v3: Add a WARN_ONCE for a missing workaround.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>