drm/radeon/kms: force legacy pll algo for RV620 LVDS

There has been periodic evidence that LVDS, on at least some
panels, prefers the dividers selected by the legacy pll algo.
This patch forces the use of the legacy pll algo on RV620
LVDS panels.  The old behavior (new pll algo) can be selected
by setting the new_pll module parameter to 1.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=30029



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