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

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 <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: [email protected]
Signed-off-by: Manasi Navare <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]