drm/nouveau: fence: fix undefined fence state after emit

nouveau_fence_emit() can fail before and after initializing the
dma-fence and hence before and after initializing the dma-fence' kref.

In order to avoid nouveau_fence_emit() potentially failing before
dma-fence initialization pass the channel to nouveau_fence_new() already
and perform the required check before even allocating the fence.

While at it, restore the original behavior of nouveau_fence_new() and
add nouveau_fence_create() for separate (pre-)allocation instead. Always
splitting up allocation end emit wasn't a good idea in the first place.
Hence, limit it to the places where we actually need to pre-allocate.

Fixes: 7f2a0b50 ("drm/nouveau: fence: separate fence alloc and emit")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230829223847.4406-1-dakr@redhat.com