drm/amdgpu: Make amdgpu_mn functions inline

Unused amdgpu_mn functions threw warnings for every file that includes
amdgpu.h. It makes sense to inline this amdgpu_mn stubs to avoid the warning.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org