radv: Add a flush postamble on GFX6.

Create a CS which contains just a cache flush,
that can be used as a postamble in command submissions.

According to RadeonSI code, the kernel flushes L2
before shaders are finished on GFX6.

Previously, RADV always added a flush at the end of
each command buffer. The flush postamble should be
a less wasteful alternative to that.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31695>

src/amd/vulkan/radv_queue.h

@@ -67,6 +67,7 @@ struct radv_queue_state {
    struct radeon_cmdbuf *continue_preamble_cs;
    struct radeon_cmdbuf *gang_wait_preamble_cs;
    struct radeon_cmdbuf *gang_wait_postamble_cs;
+   struct radeon_cmdbuf *flush_postamble_cs; /* GFX6 only */
 
    /* the uses_shadow_regs here will be set only for general queue */
    bool uses_shadow_regs;