drm/i915: Stop touching forcewake following a gen6+ engine reset

drm/i915: Stop touching forcewake following a gen6+ engine reset

Forcewake is not affected by the engine reset on gen6+. Indeed the
reason why we added intel_uncore_forcewake_reset() to
gen6_reset_engines() was to keep the bookkeeping intact because the
reset did not touch the forcewake bit (yet we cancelled the forcewake
consumers)!  This was done in commit f9f161e3f8b88:
    Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
    Date:   Fri Aug 23 16:52:30 2013 +0300

	drm/i915: sanitize forcewake registers on reset

	In reset we try to restore the forcewake state to
	pre reset state, using forcewake_count. The reset
	doesn't seem to clear the forcewake bits so we
	get warn on forcewake ack register not clearing.

That futzing of the forcewake bookkeeping was dropped in commit
54dbe639e1c7 ("drm/i915: Consolidate forcewake resetting to a single
function"), but it did not make the realisation that the remaining
intel_uncore_forcewake_reset() was redundant.

The new danger with using intel_uncore_forcewake_reset() with per-engine
resets is that the driver and hw are still in an active state as we
perform the reset. We may be using the forcewake to read protected
registers elsewhere and those results may be clobbered by the concurrent
dropping of forcewake.

Reported-by: Michel Thierry <michel.thierry@intel.com>
Fixes: 0f9eb0e20641 ("drm/i915: Modify error handler for per engine hang recovery")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817173229.20324-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>