drm/radeon/kms: fix DP after DPMS cycle

The transmitter needs to be enabled before the link is trained.

Reported-By: Lars Doelle <lars.doelle@on-line.de>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>