drm/i915/dp: Fix the enabling/disabling of audio SDP splitting

drm/i915/dp: Fix the enabling/disabling of audio SDP splitting

Adjust the enabling/disabling steps of the DP audio SDP splitting
according to a recent Bspec update. This moves the enabling to the audio
codec enable sequence after the transcoder is enabled and disables SDP
splitting explicitly during the audio disable sequence.

Bspec requires waiting for a vblank event after the transcoder is
enabled and before SDP splitting is enabled. There is no need for an
explicit wait for this, since after the transcoder is enabled this
vblank event is guaranteed to have happened via a flip done wait (see
intel_atomic_commit_tail() -> drm_atomic_helper_wait_for_flip_done()).

The bspec update is for LNL+ only, but the HW team clarified that this
has been always the intended sequence on all platforms and bspec will be
updated everywhere accordingly.

The way SDP splitting was originally enabled matched the version of
bspec at that time. Adding here the Fixes: line still, since this
change fixes a FIFO underrun on PTL during output enabling when DSC is
enabled.

Bspec: 49283, 68943
Fixes: 8853750dbad8 ("drm/i915: Enable SDP split for DP2.0")
Cc: Vinod Govindapillai <vinod.govindapillai@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250520142219.1688401-1-imre.deak@intel.com