i965/blorp/gen7+: Bring back push constant setup

i965/blorp/gen7+: Bring back push constant setup

This is partial revert of commit cc2d0e64.

It looks that even though blorp disables a stage the corresponding
3DSTATE_CONSTANT_XS packet is needed to be programmed. Hardware
seems to try to fetch the constants even for disabled stages.
Therefore care needs to be taken that the constant buffer is
set up properly. Blorp will continue to trash it into non-existing
such as before.
It is possible that this could be omitted on SKL where the
constant buffer is considered when the corresponding binding table
settings are changed. Bspec:

  "The 3DSTATE_CONSTANT_* command is not committed to the shader
   unit until the corresponding (same shader)
   3DSTATE_BINDING_TABLE_POINTER_* command is parsed."

However, as CONSTANT_XS packet itself does not seem to stall on its
own, it is safer to emit the packets for SKL also.

Possible alternative to blorp trashing could have been to setup
defaults in the beginning of each batch buffer. However, hardware
doesn't seem to tolerate these packets being programmed multiple
times per primitive. Bspec for IVB:

  "It is invalid to execute this command more than once between
   3D_PRIMITIVE commands."

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96878
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>