drm/radeon/kms: retry aux transactions if there are status flags

If there are error flags in the aux status, retry the transaction.
This makes aux much more reliable, especially on llano systems.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>