Add support to enable the DP tunnel BW allocation mode. Follow-up
patches will call the required helpers added here to prepare for a
modeset on a link with DP tunnels, the last change in the patchset
actually enabling BWA.

With BWA enabled, the driver will expose the full mode list a display
supports, regardless of any BW limitation on a shared (Thunderbolt)
link. Such BW limits will be checked against only during a modeset, when
the driver has the full knowledge of each display's BW requirement.

If the link BW changes in a way that a connector's modelist may also
change, userspace will get a hotplug notification for all the connectors
sharing the same link (so it can adjust the mode used for a display).

The BW limitation can change at any point, asynchronously to modesets
on a given connector, so a modeset can fail even though the atomic check
for it passed. In such scenarios userspace will get a bad link
notification and in response is supposed to retry the modeset.

v2:
- Fix old vs. new connector state in intel_dp_tunnel_atomic_check_state().
  (Ville)
- Fix propagating the error from
  intel_dp_tunnel_atomic_compute_stream_bw(). (Ville)
- Move tunnel==NULL checks from driver to DRM core helpers. (Ville)
- Simplify return flow from intel_dp_tunnel_detect(). (Ville)
- s/dp_tunnel_state/inherited_dp_tunnels (Ville)
- Simplify struct intel_dp_tunnel_inherited_state. (Ville)
- Unconstify object pointers (vs. states) where possible. (Ville)
- Init crtc_state while declaring it in check_group_state(). (Ville)
- Join obj->base.id, obj->name arg lines in debug prints to reduce LOC.
  (Ville)
- Add/rework intel_dp_tunnel_atomic_alloc_bw() to prepare for moving the
  BW allocation from encoder hooks up to intel_atomic_commit_tail()
  later in the patchset.
- Disable BW alloc mode during system suspend.
- Allocate the required BW for all tunnels during system resume.
- Add intel_dp_tunnel_atomic_clear_stream_bw() instead of the open-coded
  sequence in a follow-up patch.
- Add function documentation to all exported functions.
- Add CONFIG_USB4 dependency to CONFIG_DRM_I915_DP_TUNNEL.

v3:
- Rebase on intel_dp_get_active_pipes() change in previous patch.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226185246.1276018-4-imre.deak@intel.com