media: videobuf2: Remove duplicated index vs q->num_buffers check

vb2_get_buffer() already checks if the requested index is valid.
Stop duplicating this kind of check everywhere.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>