drm/msm: Don't break exclusive fence ordering

There's only one exclusive slot, and we must not break the ordering.

Adding a new exclusive fence drops all previous fences from the
dma_resv. To avoid violating the signalling order we err on the side of
over-synchronizing by waiting for the existing fences, even if
userspace asked us to ignore them.

A better fix would be to us a dma_fence_chain or _array like e.g.
amdgpu now uses, but
- msm has a synchronous dma_fence_wait for anything from another
  context, so doesn't seem to care much,
- and it probably makes sense to lift this into dma-resv.c code as a
  proper concept, so that drivers don't have to hack up their own
  solution each on their own.

v2: Improve commit message per Lucas' suggestion.

Cc: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210805104705.862416-17-daniel.vetter@ffwll.ch

drivers/gpu/drm/msm/msm_gem_submit.c

@@ -330,7 +330,8 @@ static int submit_fence_sync(struct msm_gem_submit *submit, bool no_implicit)
 				return ret;
 		}
 
-		if (no_implicit)
+		/* exclusive fences must be ordered */
+		if (no_implicit && !write)
 			continue;
 
 		ret = drm_sched_job_add_implicit_dependencies(&submit->base,