drm/radeon/kms: fix legacy LVDS dpms sequence

Add delay after turning off the LVDS encoder.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=16389



Tested-by: Jan Kreuzer <kontrollator@gmx.de>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>