Add IH function to allow caller to wait until ring entries are processed
until the checkpoint write pointer.

This will be primarily used by HMM to drain pending page fault interrupts
before memory unmap to prevent HMM from handling stale interrupts.
Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>