drm/radeon/r5xx-r7xx: wait for the MC to settle after MC blackout

Some chips seem to need a little delay after blacking out
the MC before the requests actually stop. Stop DMAR errors
reported by Shuah Khan.

Reported-by: Shuah Khan <shuahkhan@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/rv515.c

@@ -336,6 +336,8 @@ void rv515_mc_stop(struct radeon_device *rdev, struct rv515_mc_save *save)
 				WREG32(R600_CITF_CNTL, blackout);
 		}
 	}
+	/* wait for the MC to settle */
+	udelay(100);
 }
 
 void rv515_mc_resume(struct radeon_device *rdev, struct rv515_mc_save *save)