drm/i915/vrr: Allow VRR to be toggled during fastsets

Now that VRR enable/disable are called from convenient places
it is trivial to allow it to change state during fastsets.
Make it so.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7542
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320203352.19515-7-ville.syrjala@linux.intel.com
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>