drm/i915: Workaround hang with BSD and forcewake on SandyBridge

drm/i915: Workaround hang with BSD and forcewake on SandyBridge

For reasons that are not apparent to anybody, abb03c1ad32 (drm/i915:
Group the GT routines together in both code and vtable) breaks the use
of the BitStream Decoder ring on SandyBridge. The active ingredient of
that patch is the conversion from a udelay(10) to a udelay(1) in the
busy-wait loop of waiting for the forcewake acknowledge. If we restore
that udelay(10) or insert another udelay(1) afterwards (or any wait
longer than 250ns) everything works again. An alternative is also to
remove any delay from the busy-wait loop.

Given that in the atomic sections we want to complete the wait as quick
as possible to avoid blocking the CPU for too long, it makes sense to
remove the delay altogether and simply spin on the exit condition until
it completes. So we replace the udelay(1) with cpu_relax().

Papers over regression from

commit abb03c1ad32b64bee5eaf5d9b9f3dcaf524f8c99
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Jul 2 11:51:02 2012 -0300

    drm/i915: Group the GT routines together in both code and vtable

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51738
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>