drm/i915/pmu: Handle PCI unbind

Mark the device as closed and keep references to driver data alive to
allow for safe driver unbind with active PMU clients. Perf core does not
otherwise handle this case so we have to do it manually like this.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201020100822.543332-1-tvrtko.ursulin@linux.intel.com