drm/i915/vrr: Disable CMRR

Switching between variable and fixed timings is possible as for that we
just need to flip between VRR timings. However for CMRR along with the
timings, few other bits also need to be changed on the fly, which might
cause issues. So disable CMRR for now, till we have variable and fixed
timings sorted out.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311093751.1329043-5-ankit.k.nautiyal@intel.com