drm/i915: Extract intel_dbuf_bw_changed()

Extract the struct intel_dbuf_bw comparison into a small
helper. We'll get more users later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250326162544.3642-4-ville.syrjala@linux.intel.com


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