drm/i915/gt: Don't flush the tasklet if not setup

If the tasklet is not being used, don't try and flush it.

Fixes: 59489387 ("drm/i915/gt: Add a safety submission flush in the heartbeat")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200615183935.17389-1-chris@chris-wilson.co.uk