i965: Only emit VS state pipe control workaround on IVB and BYT.

i965: Only emit VS state pipe control workaround on IVB and BYT.

According to the BSpec's 3D workarounds page, this is unnecessary on
shipping Haswell hardware, and was never necessary on Broadwell.  It
unfortunately doesn't say anything about Baytrail.

The workaround database confirms those results for Ivybridge, Haswell,
and Broadwell.  Baytrail is less clear - one page says it's necessary,
while the other says it isn't.  For now, be conservative and leave it
enabled.

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