drm/i915/psr: Allow SCL=0 on platforms with always-on VRR TG

drm/i915/psr: Allow SCL=0 on platforms with always-on VRR TG

For Legacy timing generator, if there are no panel replay/sel_update or
other SRD constraints, the Set context latency (SCL) window should be
at least 1.

However, for VRR timing generator the SCL window can be 0. It has other
guardband constraints, but that are checked during guardband computation.

Allow SCL to be 0 for platforms that have VRR TG always on.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patch.msgid.link/20260517142753.2813959-3-ankit.k.nautiyal@intel.com