vaapi: move surface access function

So that everything that acceses mp_image->planes is in one place.
vaapi: remove useless cast
cosmetics
more cosmetic changes
vaapi: make image_formats allocation non-public, check error on init

No need for these functions to be public. Nothing used them, except
initialization. Also check whether the call succeeds.
vaapi: fix non-sense, minor simplification
outdated comment
vaapi: allow VO based deinterlacing using the old API even if VPP is found

We use vf_vavpp if libva has VPP support. However, using the VPP API can
still fail. This happens at least with the libva vdpau backend. So
provide at least _some_ way to let mpv use the old deinterlacer.
vaapi: document manpage