[media] media: vb2: Remove unused functions

Conversion to the use of pinned pfns made some functions unused. Remove
them. Also there's no need to lock mmap_sem in __buf_prepare() anymore.

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>