drm/i915: fixup the plane->pipe fixup code

We need to check whether the _other plane is on our pipe, not whether
our plane is on the other pipe. Otherwise if not both pipes/planes are
active, we won't properly clean up the mess and set up our desired
plane->pipe mapping.

v2: Fixup the logic, I've totally fumbled it. Noticed by Chris Wilson.

v3: I've checked Bspec, and the flexible plane->pipe mapping is a
gen2/3 feature, so test for that instead of PCH_SPLIT

v4: Check whether we indeed have 2 pipes before checking the other
pipe, to avoid upsetting i845g/i865g. Noticed by Chris Wilson.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51265
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49838


Tested-by: Dave Airlie <airlied@gmail.com>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk> #855gm
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>