drm/radeon/kms: force legacy pll algo for RV515 LVDS

drm/radeon/kms: force legacy pll algo for RV515 LVDS

commit 0d9958b18e10d7426d94cc3dd024920a40db3ee2 upstream.

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 RV515
LVDS panels.  The old behavior (new pll algo) can be selected
by setting the new_pll module parameter to 1.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>