drm/amdgpu: Clean up the register dump via debugfs list

debugfs register list for dump is cleaned as it have
some issues related to proper power state of the IP
before register read.

Since the above mentioned is removed we no longer want
this to be dumped part of the devcoredump and hence
removed.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>