drm/i915: Consolidate plane 'cleanup' operations (v3)

All plane update functions need to unpin the old framebuffer when
flipping to a new one.  Pull this logic into a separate function to ease
the integration with atomic plane helpers.

v2: Don't wait for vblank if we don't have an old fb to cleanup (Ander)

v3: Really don't wait for vblank if we don't have an old fb to cleanup.
    Previous version only handled this for primary planes; we need the
    same change on cursors/sprites too!  (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>