drm/i915: Skip some bw_state readout on pre-icl

We only compute bw_state->data_rate and bw_state->num_active_planes
on icl+. Do the same during readout so that we don't leave random
junk inside the state.

v2: Skip the whole intel_bw_crtc_update() (Vinod)

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/20250306163420.3961-13-ville.syrjala@linux.intel.com