anv: fix incorrect flushing on shader query copy

When doing query result copies in 3D mode, we're flushing the render
target cache, but the shader writes go through the dataport.

Fixes flakes/fails in piglit with shader query copies forced with Zink :

  $ query_copy_with_shader_threshold=0 ./bin/arb_query_buffer_object-coherency -auto -fbo

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: b3b12c2c ("anv: enable CmdCopyQueryPoolResults to use shader for copies")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <mesa/mesa!26797>