drm/i915: Skip repeated calls to i915_gem_set_wedged()

If we already wedged, i915_gem_set_wedged() becomes a complicated no-op.

References: https://bugs.freedesktop.org/show_bug.cgi?id=107343


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180723145335.24579-1-chris@chris-wilson.co.uk