drm/i915: Suppress assertion for i915_ggtt_disable_guc

Another step in the drv_module_reload fault-injection saga, is that we
try to disable the guc twice. Probably. It's a little unclear exactly
what is going on in the unload sequence that catches us out, so for the
time being suppress the assertion to get the test re-enabled.

Testcase: igt/drv_module_reload/basic-reload-inject
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Acked-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180720095144.5885-1-chris@chris-wilson.co.uk