drm/i915/vrr: Set vrr.enable for VRR TG with fixed_rr

drm/i915/vrr: Set vrr.enable for VRR TG with fixed_rr

For platforms that enable VRR TG only for variable timings, the
VRR_CTL.VRR_ENABLE bit indicates VRR is active. For platforms that
always have VRR TG enabled, the VRR_CTL.VRR_ENABLE bit indicates VRR
is active only when not in fixed refresh rate mode.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250324133248.4071909-8-ankit.k.nautiyal@intel.com