drm/i915: kill dev_priv->mchdev_lock

It's only ever a pointer to the global mchdev_lock, and we don't use
it at all.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>