drm/radeon/kms: prefer legacy pll algo for tv-out

ntsc seems to work fine with either algo, some
pal TVs seem pickier.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=30832



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>