drm/i915: Serialise resets with wedging

Prevent concurrent set-wedge with ongoing resets (and vice versa) by
taking the same wedge_mutex around both operations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208153708.20023-6-chris@chris-wilson.co.uk