drm/amdgpu: Quit RAS initialization earlier if RAS is disabled

If RAS is disabled through amdgpu_ras_enable kernel parameter,
we should quit the RAS initialization eariler to avoid initialization
of some RAS data structure such as sysfs etc.

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