drm/msm: avoid flood of kernel logs on faults

87e956e9 changed the fault handler to return -ENOSYS, which causes the
iommu driver to print out a huge splat.  Which wouldn't be quite so bad
if nothing ever faulted.  But seems like some EXA composite operations
generate quite a lot of (seemingly harmless) faults.  That is probably a
userspace problem, but the huge increase in verbosity from iommu fault
dumps makes things kind of unusable.

We probably should actually log *some* message (not conditional on
drm.debug).  But ratelimit it.
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/msm_iommu.c

@@ -27,8 +27,8 @@ struct msm_iommu {
 static int msm_fault_handler(struct iommu_domain *iommu, struct device *dev,
 		unsigned long iova, int flags, void *arg)
 {
-	DBG("*** fault: iova=%08lx, flags=%d", iova, flags);
-	return -ENOSYS;
+	pr_warn_ratelimited("*** fault: iova=%08lx, flags=%d\n", iova, flags);
+	return 0;
 }
 
 static int msm_iommu_attach(struct msm_mmu *mmu, const char **names, int cnt)