drm/radeon/kms: reduce probe latency on digital connectors

drm/radeon/kms: reduce probe latency on digital connectors

On a system with one HDMI and one VGA connector the latter
causes output polling to run every ten seconds. This causes
full EDID re-fetch on every poll and approx. 100ms rendering
stalls are experienced by full screen page-flipping applications.
Optimisation is to trust HPD sense on R600+ ASICs and to skip
doing these expensive probes unless HPD sense has changed.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=47007

agd5f: fix patch and message formatting.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@onelan.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>