drm/amdgpu: resolve bug in UMC 6 error counter query

iterate over all error counter registers in SMN space

removed support error counter access via MMIO

Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>