drm/i915/display: Ensure a non-zero safe window from PTL onwards

From PTL onwards, the set context latency can be in the vactive region,
letting the safe window start some lines before the vblank start. The DSB
waits on this safe window instead of vblank (bits [15:14] select the safe
window signal from the DPT unit to start the DMA engine).

With modes that have a smaller vblank region, the computed guardband is
clamped to the vblank length, making the undelayed and delayed vblank
coincide. If the SCL is also 0, the 'safe window' becomes effectively 0,
and the DSB configured to wait for it gets stalled, since the hardware
never signals the safe window, leading to:

  *ERROR* [CRTC:159:pipe A] flip_done timed out
  *ERROR* [CRTC:159:pipe A] DSB 0 timed out waiting for idle

Keep the set context latency at a minimum of 1 to avoid this.

v2: Fix the bit numbers and correct the commit message. (Chaitanya)

Fixes: 4a68c7516c57 ("drm/i915/dsb: Use safe window path when VRR TG is used")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Assisted-by: GitHub-Copilot:Claude-Opus-4.8
Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Link: https://patch.msgid.link/20260722062449.1166411-1-ankit.k.nautiyal@intel.com

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