drm/ttm: return -EBUSY if waiting for busy BO fails

Returning -EAGAIN prevents ttm_bo_mem_space from trying alternate
placements and can lead to live-locks in amdgpu_cs, retrying
indefinitely and never succeeding.

Fixes: d367bd2a ("drm/ttm: fix busy memory to fail other user v10")
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>