drm/i915: Update bw_state->active_pipes during readout

Update bw_state->active_pipes during readout.

This was completely missing from the current readout code.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306163420.3961-12-ville.syrjala@linux.intel.com