drm/i915: Fall back to bit banging mode for DVO transmitter detection

As discussed in this thread
http://lists.freedesktop.org/archives/dri-devel/2013-April/037411.html


GMBUS based DVO transmitter detection seems to be unreliable which could
result in an unusable DVO port.

The attached patch fixes this by falling back to bit banging mode for
the time DVO transmitter detection is in progress.

Signed-off-by: David Müller <d.mueller@elsoft.ch>
Tested-by: David Müller <d.mueller@elsoft.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>