i965/gen10: Ignore push constant packets during context restore.

i965/gen10: Ignore push constant packets during context restore.

These packets were causing GPU hangs when the context was restored,
possibly because they were pointing to BO's that were already
unreferenced. So we tell the hardware to ignore such packets after the
batch buffer ends, since we know those BO's are not around anymore.

This change fixes GPU hangs on CNL. The (partial) solution to this
problem so far was to entirely disable push constants on this platform.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: "18.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>