[media] media: vb2: support userptr for PFN mappings

Some video devices need to use contiguous memory which is
not backed by pages as it happens with vmalloc. This patch
provides USERPTR handling for those devices.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>