Restoration units are a fixed square size (in cm->rst_info[plane]) for
almost the entire image. The only special case is for tiles at the
right hand edge or the bottom row, which might expand or be cropped.

The av1_get_rest_ntiles function was implementing the cropping
behaviour when the image happened to be less than one restoration unit
wide or high (but not the expansion behaviour), but the result was
never useful: if you want to get the size of a restoration tile in
order to divide by it to work out what tile you're on, the fixed
square size is what you want. If you need to know how big this
particular tile is, call av1_get_rest_tile_limits.

As well as removing the output arguments from
av1_get_rest_tile_limits, this patch also removes the tile_width and
tile_height fields from the RestorationInternal structure. Note that
the tile size which is what you actually need is accessible as
rst->rsi->restoration_tilesize. (In practice, these were almost always
the same anyway).

This patch also has a couple of other small cleanups. Firstly, it
moves the subsampling_y field out of
CONFIG_STRIPED_LOOP_RESTORATION. It's not actually needed when you're
not doing striped loop restoration, but this gets rid of lots of
horrible #if/#endif lines at callsites for av1_get_rest_tile_limits.

Secondly, it simplifies the code in init_rest_search_ctxt (and fixes
some tautologous assertions). Now that YV12_BUFFER_CONFIG has a more
uniform layout, there's a simpler way to set things up, so we use
that.

Change-Id: I3c32d8ea0abe119dc86b9efa7564b27dde2151dc