drm/i915: Fix framestart_delay commens in VRR code

Since I originally wrote these comments we decided to change our
definition of framestart_delay from 0-3 to 1-4. Adjust the comments
to match that new convention. The actual code was adjusted already.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117183103.27418-3-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>