freedreno/drm/virtio: Pre-allocate cmdstream buffers

freedreno/drm/virtio: Pre-allocate cmdstream buffers

We know cmdstream buffers are immediately mmap'd, which is both
expensive on the host, and breaks the pipelining as guest is forced
to stall waiting for the host.  So pre-allocate some cmdestream
buffers, so that we have something that is (hopefully) already
allocated and mapped to guest's physical memory before we need it.
The older buffer from the head of the prealloc list replaces the
newly allocated buffer which is pushed to the tail of the prealloc
list.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19656>