drm/radeon/kms: don't disable shared encoders on pre-DCE3 display blocks

drm/radeon/kms: don't disable shared encoders on pre-DCE3 display blocks

commit a0ae5864d42b41c411368bd689462bf063c029c8 upstream.

The A/B links aren't independantly useable on these blocks so when
we disable the encoders, make sure to only disable the encoder when
there is no connector using it.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=18564

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>