r600g,radeonsi: Inform the kernel if a BO will likely be accessed by the CPU

r600g,radeonsi: Inform the kernel if a BO will likely be accessed by the CPU

This allows the kernel to prevent such BOs from ever being stored in the
CPU inaccessible part of VRAM.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>