freedreno/a6xx: WFI before RB_CCU_CNTL writes

freedreno/a6xx: WFI before RB_CCU_CNTL writes

This seems to be in a block of non buffered/context regs.  Blob always
WFIs before write, so probably a good idea.

Annoyingly, compared to ealier gens, it is a bit harder to tell from the
register offset whether it is a buffered reg, it isn't as simple as
everything below 0x2000, it seems.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>