anv/gen9: Optimize slice and subslice load balancing behavior.

anv/gen9: Optimize slice and subslice load balancing behavior.

See "i965/gen9: Optimize slice and subslice load balancing behavior."
for the rationale.  According to Jason, improves Aztec Ruins
performance by 2.7%.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
v2: Undo CPU performance micro-optimization done in i965 and iris due
    to lack of data justifying it on anv.  Use
    cmd_buffer_apply_pipe_flushes wrapper instead of emitting pipe
    control command directly.  (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>