drm/i915/dp: Allocate/free DP tunnel BW during modeset

Allocate and free the DP tunnel BW required by a stream while
enabling/disabling the stream during a modeset.

v2:
- Move the allocation up from encoder hooks to
  intel_atomic_commit_tail().
v3:
- Update the commit subject. (Ville)

Reviewed-by: Uma Shankar <uma.shankar@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240220211841.448846-17-imre.deak@intel.com