drm/amdgpu: add a flag to indicate UMC channel index version

v1 (legacy way): store channel index within a UMC instance in eeprom
v2: store global channel index in eeprom

V2: only save the flag on eeprom, clear it after saving.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>