drm/i915: Detect potential i915_reset_trylock() lockups

Use lockdep to warn before we wait indefinitely in case we may be
waiting indefinitely.

Suggested-by: Mika Kuoppala <mika.kuoppala@intel.com>
References: 2caffbf1 ("drm/i915: Revoke mmaps and prevent access to fence registers across reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190212130831.14425-2-chris@chris-wilson.co.uk