drm/nouveau: fix dma syncing for loops (v2)

The index variable should only be increased in one place.

Noticed this while trying to track down another oops.

v2: use while loop.

Fixes: f295c8cf

 ("drm/nouveau: fix dma syncing warning with debugging on.")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210311043527.5376-1-airlied@gmail.com