drm/i915/overlay: Workaround i830 overlay activation bug.

On i830, there exists a bug where an overlay on pipe B requires the mode
clock on pipe A in order to activate. So workaround this by activating
pipe A when trying to enable the overlay on pipe B.

References:

  [Bug 29007] GPU hang on video playback with overlay
  https://bugs.freedesktop.org/show_bug.cgi?id=29007



Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>