drm/i915/dp: Compute and include coasting vtotal for AS SDP

DP v2.1 allows the source to temporarily suspend Adaptive-Sync SDP
transmission while Panel Replay is active when the sink supports
asynchronous video timing.

In such cases, the sink relies on the last transmitted AS SDP timing
information to maintain the refresh rate. To support this behavior,
compute and populate the coasting vtotal field in the AS SDP payload.

Include coasting vtotal in AS SDP packing, unpacking, and comparison,
and set it during late AS SDP configuration for PR with Aux-less ALPM
when asynchronous video timing is supported.

Note:
The coasting vtotal value is fully under driver control i.e. the HW does
not overwrite these payload bytes. HW only samples the PR_ALPM_CTL[AS SDP
Transmission in Active Disable] bit during PR active state and reflects it
in the AS SDP payload at the appropriate time.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260527041050.601735-10-ankit.k.nautiyal@intel.com

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