drm/amdgpu/discovery: use hardcoded mmRCC_CONFIG_MEMSIZE

register base offset of nbio is not known before IP Discovery table is
parsed, so hardcode this value.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>