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

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>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>