i965: Make depth clear flushing more explicit

Current blorp logic issues unconditional "flush everything"
(see brw_emit_mi_flush()) after each render. For example, all
blits issue this unconditionally which shouldn't be needed if
they set render cache properly so that subsequent renders do
necessary flushing before drawing.

In case of piglit:

ext_framebuffer_multisample-accuracy all_samples depth_draw small

intel_hiz_exec() is always preceded by blorb blit and the
unconditional flush looks to hide the lack of stall and flushes
in depth clears. By removing the brw_emit_mi_flush() I get gpu
hangs.

This patch adds the stalls and flushes mandated by the spec
and gets rid of those hangs.

v2 (Jason, Ken): Document the rational for separating
                 depth cache flush and stall on Gen7.

Signed-off-by: Topi Pohjolainen <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

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