drm/i915/vrr: Add extra vblank delay to estimates

drm/i915/vrr: Add extra vblank delay to estimates

On ICL/TGL the VRR hardware injects an extra scanline just after
vactive. This essentically behaves the same as an extra line of
vblank delay, except it only appears in this one specific spot.

Consider our DSB interrupt signalling scheme:
1. arm the update
2. wait for undelayed vblank (or rather safe window with VRR)
3. wait for enough usecs to get past the delayed vblank
4. signal interrupt to indicate that arming has latched

If step 2 waits for end of vactive step 3 needs to account for
the extra one scanline, or else we risk signalling the interrupt
before the delayed vblank has actually elapsed. So include the
extra scanline in our vblank delay estimates.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241210211007.5976-16-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>