[media] media: vb2: correct queue initialization order

q->memory entry is initialized to late, so if allocation of memory buffers
fails, the buffers might not be freed correctly (q->memory is tested in
__vb2_free_mem, which can be called before setting q->memory).

Reported-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>