drm/amdgpu: cleanup creating BOs at fixed location (v2)

The placement is something TTM/BO internal and the RAS code should
avoid touching that directly.

Add a helper to create a BO at a fixed location and use that instead.

v2: squash in fixes (Alex)

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>