drm/i915: Remove pci private pointer after destroying the device private

On an aborted module load, we unwind and free our device private - but
we left a dangling pointer to our privates inside the pci_device. After
the attempted aborted unload, we may still get a call to i915_pci_remove()
when the module is removed, potentially chasing stale data.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716080332.32283-5-chris@chris-wilson.co.uk