drm/amd/display: Fix hotplug after s3 resume.

On S3 resume the HPD6 irq source was not reenabled
due to loop boundary bug.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Mikita Lipski <Mikita.Lipski@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>