drm/radeon: do a posting read in r600_set_irq

To make sure the writes go through the pci bridge.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=90741

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

drivers/gpu/drm/radeon/r600.c

@@ -3784,6 +3784,9 @@ int r600_irq_set(struct radeon_device *rdev)
 		WREG32(RV770_CG_THERMAL_INT, thermal_int);
 	}
 
+	/* posting read */
+	RREG32(R_000E50_SRBM_STATUS);
+
 	return 0;
 }