media: davinci_vpfe: remove bogus vb2->state check

There is no need to check the vb2 state in the buf_prepare
callback: it can never be wrong.

Since VB2_BUF_STATE_PREPARED will be removed in the next patch
we'll remove this unnecessary check (and use of that state) first.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>