drm/i810: off by one in i810_dma_vertex()

If vertex->idx == dma->buf_count then we end up reading one element
beyond the end of the dma->buflist[] array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703123015.kma7v7rwtdy4urce@kili.mountain