drm/i915: Set the Stencil Cache eviction policy to non-LRA mode.

Clearing bit 5 of CACHE_MODE_0 is necessary to prevent GPU hangs in
OpenGL programs such as Google MapsGL, Google Earth, and gzdoom when
using separate stencil buffers.  Without it, the GPU tries to use the
LRA eviction policy, which isn't supported.  This was supposed to be off
by default, but seems to be on for many machines.

This cannot be done in gen6_init_clock_gating with most of the other
workaround bits; the render ring needs to exist.  Otherwise, the
register write gets dropped on the floor (one printk will show it
changed, but a second printk immediately following shows the value
reverts to the old one).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47535


Cc: stable@vger.kernel.org
Cc: Rob Castle <futuredub@gmail.com>
Cc: Eric Appleman <erappleman@gmail.com>
Cc: aaron667@gmx.net
Cc: Keith Packard <keithp@keithp.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>