drm/i915: Fix PCH SSC reference clock settings

The PCH refclk settings are global, so we need to look at all of the
encoders, not just the current encoder when deciding how to configure
it. Also, handle systems with more than one panel (any combination of
PCH/non-PCH eDP and LVDS).

Disable SSC clocks when no panels are connected.

Signed-off-by: Keith Packard <keithp@keithp.com>