drm/amdgpu: rename rmn to amn in the MMU notifier code (v2)

Just a copy&paste leftover from radeon.

v2: rebase (Alex)

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>