[media] vb2: Provide helpers for mapping virtual addresses

Provide simple helper functions to map virtual address range into an
array of pfns / pages.

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>