MAINTAINERS: Remove me from amdgpu maintainers

Glad to spend time on kernel driver in past years.
I've moved to new focus in umd and couldn't commit
enough time to discussions.

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