winsys/amdgpu: do not synchronize unsynchronized buffers

winsys/amdgpu: do not synchronize unsynchronized buffers

When a buffer is added to a CS without the SYNCHRONIZED usage flag, we now
no longer add a dependency on the buffer's fence(s).

However, we still need to add a fence to the buffer during flush, so that
cache reclaim works correctly (and in the hypothetical case that the buffer
is later added to a CS _with_ the SYNCHRONIZED flag).

It is now possible that the submissions refererring to a buffer are no longer
linearly ordered, and so we may have to keep multiple fences around. We keep
the fences in a FIFO. It should usually stay quite short (# of contexts * 2,
for gfx + dma rings).

While we're at it, extract amdgpu_add_fence_dependency for a single buffer,
which will make adding the distinction between real buffer and slab cases
easier.

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