drm/i915: don't disable FBC for pipe A when flipping pipe B

Use the appropriate call.

I know there's a discussion about whether we need this call here at
all, but removing the call means we'll only update FBC after we get
the page flip IRQ. So the user may only see the new frame a little
after it should. Let's wait just a little bit more before removing
this call since we can rely in the HW tracking for accurate flips.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>