winsys/amdgpu: handle cs_add_fence_dependency for deferred/unsubmitted fences

winsys/amdgpu: handle cs_add_fence_dependency for deferred/unsubmitted fences

The idea is to fix the following interleaving of operations
that can arise from deferred fences:

 Thread 1 / Context 1          Thread 2 / Context 2
 --------------------          --------------------
 f = deferred flush
 <------- application-side synchronization ------->
                               fence_server_sync(f)
                               ...
                               flush()
 flush()

We will now stall in fence_server_sync until the flush of context 1
has completed.

This scenario was unlikely to occur previously, because applications
seem to be doing

 Thread 1 / Context 1          Thread 2 / Context 2
 --------------------          --------------------
 f = glFenceSync()
 glFlush()
 <------- application-side synchronization ------->
                               glWaitSync(f)

... and indeed they probably *have* to use this ordering to avoid
deadlocks in the GLX model, where all GL operations conceptually
go through a single connection to the X server. However, it's less
clear whether applications have to do this with other WSI (i.e. EGL).
Besides, even this sequence of GL commands can be translated into
the Gallium-level sequence outlined above when Gallium threading
and asynchronous flushes are used. So it makes sense to be more
robust.

As a side effect, we no longer busy-wait on submission_in_progress.

We won't enable asynchronous flushes on radeon, but add a
cs_add_fence_dependency stub anyway to document the potential
issue.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>