drm/i915/bw: Skip the bw_state->active_pipes update if no pipe is changing its active...

drm/i915/bw: Skip the bw_state->active_pipes update if no pipe is changing its active state

Currently we may end up doing a bunch of redundant bw_state
recomputation whenever any modeset happens. Skip a bunch of
that by only considering whether any pipe actually changes
its active state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250923171943.7319-4-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>