drm/i915/vrr: Use SCL for computing guardband

drm/i915/vrr: Use SCL for computing guardband

For now guardband is equal to the vblank length so ideally it should be
computed as difference between the vmin vtotal and vactive. However
since we are having few lines as SCL, we need to account for this while
computing the guardband.

Since the vblank start is moved by SCL lines from the vactive, the delta
between the vmin vtotal and new vblank start was used to account for this.
Now that SCL is explicitly tracked using the `set_context_latency` member,
use it directly in the guardband calculation.

In the future, when the guardband is shortened or optimized, we may need
to factor in both the change in the vblank start and SCL lines. For now,
explicitly accounting for SCL is sufficient.

v2: Fix typo: replace adjusted_mode->vdisplay with
adjusted_mode->crtc_vdisplay. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250924141542.3122126-5-ankit.k.nautiyal@intel.com