drm/i915/dp: Add a helper to decide if AS SDP can be used

drm/i915/dp: Add a helper to decide if AS SDP can be used

Add a helper that determines whether AS SDP can be used for the
current DP configuration. For now this is true only when the sink
supports AS SDP and VRR is enabled, but more conditions may be added
later.

v2:
 - Rename to intel_dp_needs_as_sdp(). (Ville)
 - Add a #FIXME documenting non-atomic of DP SDP updates. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260424090942.3060291-3-ankit.k.nautiyal@intel.com