drm/i915/pmu: Keep a reference to module while active

While a perf event is open, keep a reference to the module so we don't
remove the driver internals mid-sampling.

Testcase: igt/perf_pmu/module-unload
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200430183324.23984-1-chris@chris-wilson.co.uk