drm/i915: Fix vblank evasion with vrr

With vrr enabled the hardware no longer latches the registers
automagically at vblank start. The point at which it will do the
latching even when no push has been sent is the vmax decision
boundary. That is the thing we need to evade to avoid our
register latching to get split between two frames.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122232647.22688-18-manasi.d.navare@intel.com