drm/i915: VGA needs to be on pipe A on i830M

The bit doesn't stick, and the output is always cloned from pipe A,
even when it's supposed to scan out from pipe B.

Shuts up annoying warnings from the modeset-rework, too.

I've noticed that with this patch we know get and unknown connection
state since the code can't find a suitable pipe for load detection.
But that beats the previous state of affairs, where it tried to use
pipe B, actually used pipe A and concluded that something is connected
(although it's the LVDS on pipe A and nothing on the VGA connector on
pipe B).

I've tried to make load detect work by remapping the pipe->planes
stuff, so that crtc 0 will use pipe B and hence we still have
something left for load-detect on pipe A. But alas, that upset the hw
a bit.

So there's still some things to figure out, but this here will at
least paper over some of the problems.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51265


Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: extend the commit message a bit with recent observations.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>