[media] media: vb2: Convert vb2_dc_get_userptr() to use frame vector

Convert vb2_dc_get_userptr() to use frame vector infrastructure. When we
are doing that there's no need to allocate page array and some code can
be simplified.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>