drm/amdgpu: add graphic memory controller implementation for si v7

v4: rebase fixups
v5: rebase fixups
v5: rebase fixups
v6: rebase fixups for gart size changes
v7: add gmc v6 firmware versions for sysfs dump

Signed-off-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>