drm/i915/display: Fix inverted WARN_ON

Restore the previous WARN_ON(cond) so that we don't complain about poor
old Cherryview.

Fixes: eb020ca3

 ("drm/i915/display/dp: Make WARN* drm specific where drm_device ptr is available")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200223173959.3885742-1-chris@chris-wilson.co.uk