drm/amdgpu: enable GTT PD/PT for raven v3

Should work on Vega10 as well, but with an obvious performance hit.

Older APUs can be enabled as well, but will probably be more work.

v2: fix error checking
v3: use more general check

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>