drm/amdgpu: add the lost mutex_init back

Initialize notifier_lock.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/1016


Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>