radeonsi: only add descriptors in presence of resident handles

radeonsi: only add descriptors in presence of resident handles

This won't help much except for applications that use a ton
of resident handles. Though, this will reduce the winsys
overhead a little bit.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>