drm/i915/guc: Ensure H2G buffer updates visible before tail update

Ensure H2G buffer updates are visible before descriptor tail updates by
inserting a barrier between the H2G buffer update and the tail. The
barrier is simple wmb() for SMEM and is register write for LMEM. This is
needed if more than 1 H2G can be inflight at once.

If this barrier is not inserted it is possible the descriptor tail
update is scene by the GuC before H2G buffer update which results in the
GuC reading a corrupt H2G value. This can bring down the H2G channel
among other bad things.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-16-matthew.brost@intel.com