drm/radeon/kms: Fix logic error in DP HPD handler

commit 5ba7ddf8 upstream.

Only disable the pipe if the monitor is physically
disconnected.  The previous logic also disabled the
pipe if the link was trained.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=41248



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>