drm/radeon/kms: Disable TV load detect on RS400,RC410,RS480

RS400,RC410,RS480 chipset seems to report a lot of false positive
with load detect on TV output. We haven't yet found a way to make
load detect reliable on those chipset, thus just disable it for TV
output. Would avoid user to experience phantom screen because X
believe there is a monitor connected to the TV output.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>