Add the helper function _get_surface_id() which extracts the
VASurfaceID from the passed picture. This function gets the surface of
the next and previous reference picture.

Instead of if-statements, this refactor uses a switch-statement with a
fall-through, for P-type pictures, making the code a bit more readable.

Also it adds quirks for gallium driver, which cannot handle invalid
surfaces as forwarding nor backwarding references, so the function fails.
Also iHD cannot handle them, but to avoid failing, the current picture
is used as self-reference.

Part-of: <!1939>