drm/i915: Apply HWSTAM workaround for BSD ring on SandyBridge

...we need to apply exactly the same workaround for missing interrupts
from BSD as for the BLT ring, apparently.

See also commit 498e720b
(drm/i915: Fix gen6 (SNB) missed BLT ring interrupts).

Reported-and-tested-by:  <nkalkhof@web.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38529


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>