drm/amdgpu: decouple EccErrCnt query and clear operation

Due to hardware bug that when RSMU UMC index is disabled,
clear EccErrCnt at the first UMC instance will clean up all other
EccErrCnt registes from other instances at the same time. This
will break the correctable error count log in EccErrCnt register
once querying it. So decouple both to make error count query workable.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>