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:
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>