drm/i915: WARN if power sequencer is not connected to the LVDS port on pre-ilk

We don't support using the power sequencer with other ports besides LVDS
on pre-ilk platforms. WARN if someone has mistakenly connected the power
sequencer to the wrong port.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518152931.13104-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>