drm/i915: improve assert_panel_unlocked

Fix assert_panel_unlocked for vlv/chv, and improve it a bit for
non-LVDS. Also don't pretend it works for DDI. There's still work to do
to get this right for eDP on PCH platforms, but this is a start.

v2: WARN_ON(HAS_DDI)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>