drm/i915: Double check bumping after the spinlock

In preparation for making GEM execbuf parallel, we need to be prepared
to handle very early declaration of dependencies -- even before our
signaler has itself been submitted.

References: a79ca656 ("drm/i915: Push the wakeref->count deferral to the backend")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200220123608.1666271-1-chris@chris-wilson.co.uk