radv/winsys: fix leaking resources from bo's imported by fd

A bo's ref_count was not being initialized when imported from an fd.
Therefore, we would fail to free the resource during VkFreeMemory().

This patch fixes applications like hifi VR in threaded mode, which
perform frequent imports/releases of IPC shared memory.

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
CC: 18.0 18.1 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit f56e22e4)