iris: Allow slab-allocation of explicitly-coherent buffers
This series fixes iris to use the suballocator for temporary staging blits, among other cases, on all LLC or discrete hardware.
This makes 95% of the BO allocations in a Dota2 trace hit the suballocator path, rather than being allocated from real BOs. This should help with an issue Felix noticed where we were calling GEM_CREATE and MMAP_OFFSET too frequently.