anv/cmd_buffer: Only emit PIPE_CONTROL on-demand

anv/cmd_buffer: Only emit PIPE_CONTROL on-demand

This is in contrast to emitting it directly in vkCmdPipelineBarrier.  This
has a couple of advantages.  First, it means that no matter how many
vkCmdPipelineBarrier calls the application strings together it gets one or
two PIPE_CONTROLs.  Second, it allow us to better track when we need to do
stalls because we can flag when a flush has happened and we need a stall.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>