drm/amdgpu: preserve RSMU UMC index mode state

between UMC RAS err register access restore previous RSMU UMC index mode state

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>