llvmpipe: shortcircuit repeated lookups of the same tile

llvmpipe: shortcircuit repeated lookups of the same tile

The lp_tile_cache is often called repeatedly to look up the same
tile.  Add a cache (to the cache) of the single tile most recently
retreived and make a quick inline check to see if this matches the
subsequent request.

Add a tile_address bitfield struct to make this check easier.