drm/i915: fix DDI get_hw_state return value

If we couldn't find a pipe we shouldn't return true.  This might be even
better as a WARN though, since it should be impossible to have the port
enabled without a pipe selected.

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