It's okay to use bricks as fill, but I am guessing that the arrangement you are planning may not be the best. The concrete won't flow around the bricks, and they could move, eventually causing cracking of the stuff above. Normally for a base, you'd pack it down (using something like gravel), but the bricks won't pack, nor are they likely perfectly flat, and would start out with a somewhat unstable (from a tile's viewpoint) base. I think you could get by with one layer, spaced a bit apart so you'd get the concrete to go between them. See what others have to say, as I'm guessing here.
Or, you could essentially 'set' the bricks with some of the concrete acting as mortar, then pour in the rest to fill the form up...that would make it monolithic the full depth rather than a floating layer on a potentially unstable base.