drm/amdgpu: skip reservation of discovery tmr region in pre-Navi

IP discovery is only supported in Navi series and onwards.
There is no need to reserve a portion of vram as discovery
tmr region for pre-Navi adapters.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>