drm/i915: Enable provoking vertex fix on Gen9 systems.

The SF and clipper units mishandle the provoking vertex in some cases,
which can cause misrendering with shaders that use flat shaded inputs.

There are chicken bits in 3D_CHICKEN3 (for SF) and FF_SLICE_CHICKEN
(for the clipper) that work around the issue.  These registers are
unfortunately not part of the logical context (even the power context),
and so we must reload them every time we start executing in a context.

Bugzilla: https://bugs.freedesktop.org/103047
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180615190605.16238-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: stable@vger.kernel.org