[media] videobuf2-core: print current state of buffer in vb2_buffer_done

In vb2_buffer_done, it would be better the print the value of 'state'
(current state of buffer) than to print 'vb->state' which is always
VB2_BUF_STATE_ACTIVE.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>