drm/radeon: fix HD6790, HD6570 backend programming

Without this bit sets we get broken rendering and
lockups.

fglrx sets this bit.

Bugs that should be fixed by this patch :
https://bugs.freedesktop.org/show_bug.cgi?id=49792
https://bugzilla.kernel.org/show_bug.cgi?id=43207
https://bugs.freedesktop.org/show_bug.cgi?id=39282

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/evergreen.c

@@ -2178,9 +2178,9 @@ static void evergreen_gpu_init(struct radeon_device *rdev)
 		WREG32(CC_SYS_RB_BACKEND_DISABLE, rb);
 		WREG32(GC_USER_RB_BACKEND_DISABLE, rb);
 		WREG32(CC_GC_SHADER_PIPE_CONFIG, sp);
-        }
+	}
 
-	grbm_gfx_index |= SE_BROADCAST_WRITES;
+	grbm_gfx_index = INSTANCE_BROADCAST_WRITES | SE_BROADCAST_WRITES;
 	WREG32(GRBM_GFX_INDEX, grbm_gfx_index);
 	WREG32(RLC_GFX_INDEX, grbm_gfx_index);