gallium, radeonsi: allow allocating planar video formats via resource_create
The motivation is to allocate multiple planes in 1 backing buffer via
resource_create. For example, if you pass PIPE_FORMAT_NV12 into resource_create, you'll get 2 chained pipe_resources, R8 and R8G8.
pipe_video_buffer can be implemented using a new helper
vl_video_buffer_create_as_resource, which just calls
si_vid_join_surfaces nonsense is removed.