drm/i915/display: VRR + DRRS cannot be enabled together

If VRR is enabled, DRRS cannot be enabled, so make this check
in atomic check.

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122232647.22688-8-manasi.d.navare@intel.com