drm/i915: Move dbuf_state->active_pipes into skl_wm_get_hw_state()

Move the dbuf_state readout parts into skl_wm_get_hw_state()
so that the details are better hidden from sight.

This will stop updating this on pre-skl, but that's what we want
since the dbuf state is only used on skl+.

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/20250306163420.3961-18-ville.syrjala@linux.intel.com