drm/i915/vrr: Fix "window2" handling

The "window2" delay is just the difference of vactive
(undelayed vblank) vs. vblank_start (delayed vblank).
Just use vblank_start during the VRR calculations so
that things work correctly regardless of whether delayed
vblank is used or not.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230127173044.24108-2-ville.syrjala@linux.intel.com


Reviewed-by: Jouni Högander <jouni.hogander@intel.com>