intel/fs/ra: Pull the guts of RA into its own class

intel/fs/ra: Pull the guts of RA into its own class

This accomplishes two things.  First, it makes interfaces which are
really private to RA private to RA.  Second, it gives us a place to
store some common stuff as we go through the algorithm.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>