drm/i915/pmu: Initialise the spinlock before registering

As the GT may be running in parallel with the module initialisation
code, we may enter i915_pmu_gt_parked() as we are executing
i915_pmu_register(). We have to init the spinlock before we mark
pmu.event_init so that it is available for use by i915_pmu_gt_parked()
(which may run as soon as event_init is set).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112127


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025165442.23356-1-chris@chris-wilson.co.uk