drm/ttm: cleanup and optimize ttm_bo_mem_compat v2

No need to implement the same logic twice. Also check if the busy placements
are identical to the already scanned placements before checking them.

v2: improve check even more as suggested by Michel.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>