v4l2bufferpool: fix import_userptr() in single-planar API when n_planes > 1

v4l2bufferpool: fix import_userptr() in single-planar API when n_planes > 1

In the V4L2 single-planar API, when format is semi-planar/planar,
drivers expect the planes to be contiguous in memory.
So this commit change the way we handle semi-planar/planar format
(n_planes > 1) when we use the single-planar API (group->n_mem == 1).

To check that planes are contiguous and have expected size, ie: no
padding. We test the fact that plane 'i' start address + plane 'i'
expected size equals to plane 'i + 1' start address. If not, we return
in error.

Math are done in bufferpool rather than in allocator because the
former is aware of video info.

https://bugzilla.gnome.org/show_bug.cgi?id=738013