drm/radeon: fix & improve ih ring handling v3

The spinlock was actually there to protect the
rptr, but rptr was read outside of the locked area.

Also we don't really need a spinlock here, an
atomic should to quite fine since we only need to
prevent it from being reentrant.

v2: Keep the spinlock....
v3: Back to an atomic again after finding & fixing the real bug.

Signed-off-by: Christian Koenig <christian.koenig@amd.com>