drm/i915: Fix possible overflow when recording semaphore states.

semaphore _sync_seqno, _seqno and _mbox are smaller than number of rings.
This optimization is to remove the ring itself from the list and the logic to do that
is at intel_ring_sync_index as below:

/*
 * rcs -> 0 = vcs, 1 = bcs, 2 = vecs, 3 = vcs2;
 * vcs -> 0 = bcs, 1 = vecs, 2 = vcs2, 3 = rcs;
 * bcs -> 0 = vecs, 1 = vcs2. 2 = rcs, 3 = vcs;
 * vecs -> 0 = vcs2, 1 = rcs, 2 = vcs, 3 = bcs;
 * vcs2 -> 0 = rcs, 1 = vcs, 2 = bcs, 3 = vecs;
*/

v2: Skip when from == to (Damien).
v3: avoid computing idx when from == to (Damien).
    use ring == to instead of ring->id == to->id (Damien).
    use continue instead of return (Rodrigo).
v4: avoid all unecessary computation (Damien).
    reduce idx to loop scope (Damien).

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>