drm/amdkfd: Drop interrupt SW ring buffer

The work queue couldn't reliably prevent the SW ring buffer from
overflowing, so dmesg was spammed by

 kfd kfd: Interrupt ring overflow, dropping interrupt.

messages when running e.g. the Atlantis Substance demo from
https://wiki.unrealengine.com/Linux_Demos

 on Kaveri.

Since the SW ring buffer doesn't actually do anything at this point, just
remove it for now. When actual interrupt processing code is added to
amdkfd, it should try to do things immediately and only defer to work
queues when necessary.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>