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 498e720b96379d8ee9c294950a01534a73defcf3
(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>