i965: Add PIPE_CONTRTOL_DATA_CACHE flush to brw_emit_mi_flush().

i965: Add PIPE_CONTRTOL_DATA_CACHE flush to brw_emit_mi_flush().

Although we're phasing out brw_emit_mi_flush(), we still use it in some
places in order to "flush everything".  In a number of those places, we
write data to a buffer that we may then bind as an image surface, SSBO,
or atomic buffer.  Those usages require us to flush the data cache.

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