g4x dplls and ilk+ pch plls have a separate field for the reduced p1
setting, so this restriction does not apply. Only older platforms have
the restriction that the p1 divisors must match.

This unnecessary restriction has been introduced in

commit cec2f356
Author: Sean Paul <seanpaul@chromium.org>
Date:   Tue Jan 10 15:09:36 2012 -0800

    drm/i915: Only look for matching clocks for LVDS downcloc

Note that with lvds the p2 divisors _always_ match for LVDS, and we
don't support auto-downclocking anywhere else. On eDP downclocking
works with separate data m/n settings, using the same link clock.

Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>