drm/i915: fixup assert_pipe to take the pipe A quirk into account

This was completely spamming dmesg on my i855gm. This issue was just
shortly introduced with:

commit 931872fc
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Jan 16 23:01:13 2012 +0000

    drm/i915: Check that plane/pipe is disabled before removing the fb

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>