drm/i915/vrr: Remove unwanted comment

The comment about fixed average vtotal is incorrect.
Remove it.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311093751.1329043-2-ankit.k.nautiyal@intel.com