drm/radeon/kms: fix up LVDS handling on macs (v2)

Based on radeonfb code and recent ddx fix.

v2: minor formatting fix from Michel Dänzer

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Michel Dänzer <michel@daenzer.net>
Tested-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>