[media] vb2: don't set index, don't start streaming for write()

Two fixes:

- there is no need to set the index when calling dqbuf: dqbuf will
  overwrite it.
- __vb2_init_fileio already starts streaming for write(), so there is
  no need to do it again in __vb2_perform_fileio. It can never have
  worked anyway: either __vb2_init_fileio succeeds in starting streaming
  or it is never going to happen.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>