drm/amdgpu: fix holding mn_lock while allocating memory

We can't hold the mn_lock while allocating memory.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>