On some chipset we try to avoid possibly invasive output detection
methods (like load detect which can cause flickering elsewhere) in the
output poll work. Drivers could hence return unknown when a previous
full ->detect call returned a different state.

This change will generate a hotplug event, forcing userspace to do a
full scan. This in turn updates the connector->status field so that we
will _again_ get a state change when the hotplug work re-runs in 10
seconds.

To avoid this ping-pong loop detect this situation and clamp the
connector state to the old value.

Patch is inspired by a patch from Knut Peterson. Knut's patch
completely ignored connector state changes if either the old or new
status was unknown, which seemed to be a bit too agressive to me.

v2: Rebased onto the drm_probe_helper.c extraction.

References: http://lists.freedesktop.org/archives/dri-devel/2012-August/025975.html
Cc: Knut Petersen <Knut_Petersen@t-online.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>