drm/amdgpu: fix typo in amdgpu_mn.c comments

In doc comments for struct amdgpu_mn: destrution -> destruction

Signed-off-by: Slava Abramov <slava.abramov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>