This allows most GPU objects to use the full 48-bit address space
offered by Gen8+ platforms, rather than being stuck with 32-bit.
This expands the available GPU memory from 4G to 256TB or so.

A few objects - instruction, scratch, and vertex buffers - need to
remain pinned in the low 4GB of the address space for various reasons.
We default everything to 48-bit but disable it in those cases.

Thanks to Jason Ekstrand for blazing this trail in anv first and
finding the nasty undocumented hardware issues.  This patch simply
rips off all of his findings.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>