drm/i915/selftests: Always call the provided engine->emit_init_breadcrumb

While this does not appear to fix any issues, the backend itself knows
when it wants to emit a breadcrumb, so let it make the final call.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513074809.18194-16-chris@chris-wilson.co.uk