drm/radeon: do a posting read in rs600_set_irq

commit 54acf107 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>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>

drivers/gpu/drm/radeon/rs600.c

@@ -585,6 +585,10 @@ int rs600_irq_set(struct radeon_device *rdev)
 	WREG32(R_006540_DxMODE_INT_MASK, mode_int);
 	WREG32(R_007D08_DC_HOT_PLUG_DETECT1_INT_CONTROL, hpd1);
 	WREG32(R_007D18_DC_HOT_PLUG_DETECT2_INT_CONTROL, hpd2);
+
+	/* posting read */
+	RREG32(R_000040_GEN_INT_CNTL);
+
 	return 0;
 }