drm/i915/dp_tunnel: Split update_tunnel_state()

drm/i915/dp_tunnel: Split update_tunnel_state()

Split update_tunnel_state() into two helpers: one that updates the
tunnel state, and another that detects whether the tunnel bandwidth
has changed.

This prepares for a follow-up change that needs to compare the current
bandwidth against the value from before the DP tunnel was detected and
bandwidth allocation mode was enabled.

While at it, document the return value of update_tunnel_state().

Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260219182823.926702-4-imre.deak@intel.com