i965: Drop VECTOR_MASK_ENABLE in Broadwell's 3DSTATE_VS packet.

i965: Drop VECTOR_MASK_ENABLE in Broadwell's 3DSTATE_VS packet.

We never set it on previous generations, but I had to set it in
3DSTATE_PS for correct behavior.  For symmetry, I set it in 3DSTATE_VS
as well, but there's no actual need to do so.  Piglit works fine either
way.  The documentation also remarks that there should never be a need
to program this.

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