drm/vrr: Set VRR capable prop only if it is attached to connector

VRR capable property is not attached by default to the connector
It is attached only if VRR is supported.
So if the driver tries to call drm core set prop function without
it being attached that causes NULL dereference.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225013055.9282-1-manasi.d.navare@intel.com