drm/i915/dp_tunnel: Send BW change notification after tunnel creation

drm/i915/dp_tunnel: Send BW change notification after tunnel creation

Detecting a bandwidth change for a sink connected through a DP tunnel
depends on updating the sink's DPRX link rate and lane count.

detect_new_tunnel() -> update_tunnel_state() updates the link
configuration only if the tunnel state changes. However, after the
tunnel is created and bandwidth allocation mode is enabled, the tunnel
state itself may remain unchanged.

Record the sink bandwidth before creating the tunnel and compare it to
the bandwidth after tunnel creation and enabling bandwidth allocation
mode, ensuring that any bandwidth change is detected and userspace is
notified accordingly.

Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260219182823.926702-6-imre.deak@intel.com