drm/i915/gt: Remember to free the virtual breadcrumbs

Since we allocate some breadcrumbs for the virtual engine, and the
virtual engine has a custom destructor, we also need to free the
breadcrumbs after use.

Fixes: b3786b29

 ("drm/i915/gt: Distinguish the virtual breadcrumbs from the irq breadcrumbs")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201118133839.1783-1-chris@chris-wilson.co.uk