drm/i915/display: Add helper for AS SDP transmission time selection

drm/i915/display: Add helper for AS SDP transmission time selection

AS SDP may be transmitted at T1 or T2 depending on Panel Replay and
Adaptive Sync SDP configuration as per DP 2.1. Current we are using
T1 only, but future PR/AS SDP modes/features may require T2 or dynamic
selection.

Introduce a helper to return the appropriate AS SDP transmission time so
that a single value is consistently used for programming PR_ALPM.
For now this returns T1.

v2: Avoid adding new member to crtc_state; use a helper. (Ville)
v3: Clarify why AS SDP transmission time is fixed to T1. (Ville)
v4: Return u8 from intel_dp_as_sdp_transmission_time(). (Ville)

Bspec: 68920
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260527041050.601735-6-ankit.k.nautiyal@intel.com