drm/radeon/evergreen+: 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.

May fix:
https://bugs.freedesktop.org/show_bug.cgi?id=56139
https://bugs.freedesktop.org/show_bug.cgi?id=57567

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/evergreen.c

@@ -1349,6 +1349,8 @@ void evergreen_mc_stop(struct radeon_device *rdev, struct evergreen_mc_save *sav
 		blackout &= ~BLACKOUT_MODE_MASK;
 		WREG32(MC_SHARED_BLACKOUT_CNTL, blackout | 1);
 	}
+	/* wait for the MC to settle */
+	udelay(100);
 }
 
 void evergreen_mc_resume(struct radeon_device *rdev, struct evergreen_mc_save *save)