radv: Emit pending flushes before executing a secondary command buffer

radv: Emit pending flushes before executing a secondary command buffer

If we have any pending flushes on the primary command buffer, these
must be performed before executing the secondary buffer.

This fixes potential corruption when the contents of a subpass which
clears any of its render targets are given in a secondary buffer: the
flushes after a fast clear would not have been performed until the
vkCmdEndRenderPass call.

Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: 13.0 17.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 290d7e892dfa6d04767142f4f6d7ec689933a105)