drm/radeon: do a posting read in r600_set_irq

commit 9d1393f2 upstream.

To make sure the writes go through the pci bridge.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=90741Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/radeon/r600.c

@@ -3156,6 +3156,9 @@ int r600_irq_set(struct radeon_device *rdev)
 		WREG32(DC_HOT_PLUG_DETECT3_INT_CONTROL, hpd3);
 	}
 
+	/* posting read */
+	RREG32(R_000E50_SRBM_STATUS);
+
 	return 0;
 }