drm/i915: Don't do PM5/DDR DVFS with multiple pipes

Enabling PM5/DDR DVFS with multiple active pipes isn't a validated
configuration. It does seem to work most of the time at least, but
there is clearly an additional risk of underruns, so let's not play
with fire.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>