drm/i915: Extract intel_bw_modeset_checks()

Pull the new_bw_state->active_pipes computation out from
intel_compute_sagv_mask() and move it into the intel_bw.c
(which is arguably the correct place for it).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250326162544.3642-10-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>