check UMC status and exit prior to making and erroneus register access

this resolved unexpected behaviour with UMC indexing mode broadcasting writes
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>