We had a, what was supposed to be temporary, hack in the KMS code where we'd
completely drain an EVO/NVD channel's push buffer when wrapping to the start
again, instead of treating it as a ring buffer.
Let's fix that, finally.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
This will be required to support Volta, but also allows us to remove code
that's duplicated for each channel type already.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
