drm/i915/vrr: Fix guardband/vblank exit length calculation for adl+

drm/i915/vrr: Fix guardband/vblank exit length calculation for adl+

We are miscalculating both the guardband value, and the resulting
vblank exit length on adl+. This means that our start of vblank
(double buffered register latch point) is incorrect, and we also
think that it's not where it actually is (hence vblank evasion/etc.
may not work properly). Fix up the calculations to match the real
hardware behaviour (as reverse engineered by intel_display_poller).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221202134412.21943-3-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>