drm/i915: fix up the IBX transcoder B check

This has been added in

commit de9a35ab
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jun 5 11:03:40 2012 +0200

    drm/i915: assert that the IBX port transcoder select w/a is implemented

Unfortunately I've failed to notice that these checks are not just
called for the port that is about to be disabled, but for all (which
makes sense for an assert ...), and the WARN missfired when disabling
another pipe than the one with the dp port.

Hence also check whether the port is actually disabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54688


Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>