drm/ttm: return -EBUSY on pipelining with no_gpu_wait (v2)

Setting the no_gpu_wait flag means that the allocate BO must be available
immediately and we can't wait for any GPU operation to finish.

v2: squash in mem leak fix, rebase

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>