drm/i915: Extract intel_bw_update_hw_state()

Hoist the bw stuff into a separate function from
intel_modeset_readout_hw_state() so that the details
are better hidden inside intel_bw.c.

We can also skip the whole thing on pre-skl since the dbuf state
isn't actually used on those platforms.

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/20250306163420.3961-11-ville.syrjala@linux.intel.com