anv: Advertise larger heap sizes

anv: Advertise larger heap sizes

Instead of just advertising the aperture size, we do something more
intelligent.  On systems with a full 48-bit PPGTT, we can address 100%
of the available system RAM from the GPU.  In order to keep clients from
burning 100% of your available RAM for graphics resources, we have a
nice little heuristic (which has received exactly zero tuning) to keep
things under a reasonable level of control.

Reviewed-by: Kristian H. Kristensen <krh@bitplanet.net>