drm/i915/dp: Suspend/resume DP tunnels

Suspend and resume DP tunnels during system suspend/resume, disabling
the BW allocation mode during suspend, re-enabling it after resume. This
reflects the link's BW management component (Thunderbolt CM) disabling
BWA during suspend. Before any BW requests the driver must read the
sink's DPRX capabilities (since the BW manager requires this
information, so snoops for it on AUX), so ensure this read takes place.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
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/20240220211841.448846-20-imre.deak@intel.com