gallium: add PIPE_FLUSH_{TOP,BOTTOM}_OF_PIPE bits

gallium: add PIPE_FLUSH_{TOP,BOTTOM}_OF_PIPE bits

These bits are intended to be used by the ddebug hang detection and are
named in analogy to the Vulkan stage bits (and the corresponding Radeon
pipeline event).

Hang detection needs fences on the granularity of individual commands,
which nothing else really covers. The closest alternative would have
been PIPE_QUERY_GPU_FINISHED, but (a) queries are a per-context object
and we really want a per-screen object, (b) queries don't offer a
wait with timeout, and (c) in any case, PIPE_QUERY_GPU_FINISHED is
meant to imply that GPU caches are flushed, which the new bits
explicitly aren't.

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