drm/i915: Steal power sequencer in vlv_power_sequencer_pipe()

In case we fumble something and end up picking an already used power
seqeuencer in vlv_power_sequencer_pipe() at least try to steal it
gracefully. In theory this should never happen though.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>