drm/i915/vbt: Add VRR VBT toggle

This will be used in future but already adding to VBT so we are
updated with VBT changes.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201008211932.24989-3-jose.souza@intel.com