drm/i915: Swap primary planes on gen2 for FBC

Only plane A is FBC capable on gen2 (like gen3), but the panel fitter
is hooked up to pipe B, so we want to prefer pipe B + plane A.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add the code comment Chris requested in his review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>