drm/amdgpu: Delete some cgs functions

Drop cgs wrappers that are no longer used.
1. cgs_rel_firmwar
2. cgs_is_virtualization_enabled
3. cgs_notify_dpm_enabled
4. cgs_atom_get_data_table
5. cgs_atom_get_cmd_table_revs
6. cgs_atom_exec_cmd_table
7. cgs_get_active_displays_info

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>