drm/i915: Remove some duplicated plane swapping logic

On pre-gen4 we connect plane A to pipe B and vice versa to get an FBC
capable plane feeding the LVDS port by default. We have the logic for
the plane swapping duplicated in many places. Let's remove a bit of the
duplication by having the crtc look up the thing from the primary plane.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478616439-10150-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>