[media] ivtv: DECODER_CMD v4l2-compliance fixes

VIDIOC_DECODER_CMD didn't return EPERM when calling PAUSE or RESUME if no
decoding is in progress.
VIDIOC_G_ENC_INDEX didn't set entries_cap or return 0 if no decoding was
in progress.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>