drm/amdkfd: use standard kernel kfifo for IH

Replace our implementation of a lockless ring buffer with the standard
linux kernel kfifo.

We shouldn't maintain our own version of a standard data structure.

Signed-off-by: Andres Rodriguez <andres.rodriguez@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>