venus: place wsi submissions on corresponding ring_idx

venus: place wsi submissions on corresponding ring_idx

With implicit fencing, the image has a fence that blocks scanout until
rendering is complete. virtgpu doesn't support implicit fencing yet, but
Sommelier (a VM Wayland compositor) does the wait by exposing the bo as
a GEM handle and waiting on all fences in userspace with a
DRM_IOCTL_VIRTGPU_WAIT before issuing the wl_surface commit.
During vkQueueSubmit involving wsi images, we follow with an empty
renderer submission on the corresonding ring_idx to install a fence
on the appropriate virtgpu fence context after the last rendering
submission.

Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19691>