On gfx943 APU, EXT_COHERENT should give MTYPE_CC for local and
MTYPE_UC for nonlocal memory.

On NUMA systems, local memory gets the local mtype, set by an
override callback. If EXT_COHERENT is set, memory will be set as
MTYPE_UC by default, with local memory MTYPE_CC.

Add an option in the override function for this case, and
add a check to ensure it is not used on UNCACHED memory.

V2: Combined APU and NUMA code into one patch
V3: Fixed a potential nullptr in amdgpu_vm_bo_update
Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>