drm/i915/pps: move vlv_active_pipe() to intel_pps.c

drm/i915/pps: move vlv_active_pipe() to intel_pps.c

All the users for vlv_active_pipe() are within intel_pps.c now, and
there are already uses of g4x_dp_port_enabled() and intel_dp->output_reg
in there, so seems fine to reduce interfaces and move vlv_active_pipe()
to intel_pps.c too.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e12ae0a931f113f3bbbf1b4c66108b572a933efb.1726681620.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>