drm/i915: Guard against i915_ggtt_disable_guc() being invoked unconditionally

Commit 7c3f86b6 ("drm/i915: Invalidate the guc ggtt TLB upon
insertion") added the restoration of the invalidation routine after the
GuC was disabled, but missed that the GuC was unconditionally disabled
when not used. This then overwrites the invalidate routine for the older
chipsets, causing havoc and breaking resume as the most obvious victim.

We place the guard inside i915_ggtt_disable_guc() to be backport
friendly (the bug was introduced into v4.11) but it would be preferred
to be in more control over when this was guard (i.e. do not try and
teardown the data structures before we have enabled them). That should
be true with the reorganisation of the guc loaders.
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 7c3f86b6 ("drm/i915: Invalidate the guc ggtt TLB upon insertion")
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: <stable@vger.kernel.org> # v4.11+
Link: http://patchwork.freedesktop.org/patch/msgid/20170531190514.3691-1-chris@chris-wilson.co.ukReviewed-by: Michel Thierry <michel.thierry@intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3100,6 +3100,7 @@ void i915_ggtt_enable_guc(struct drm_i915_private *i915)
 
 void i915_ggtt_disable_guc(struct drm_i915_private *i915)
 {
+	if (i915->ggtt.invalidate == guc_ggtt_invalidate)
 		i915->ggtt.invalidate = gen6_ggtt_invalidate;
 }