drm/i915/gt: Avoid using tag 0 for the very first submission

Assume that the HW starts off with tag 0 "active" and so avoid using tag
0 for our own first ELSP submission.

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/20191229183153.3719869-2-chris@chris-wilson.co.uk