drm/radeon/kms: clean up some magic numbers

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>
diff --git a/drivers/gpu/drm/radeon/r520.c b/drivers/gpu/drm/radeon/r520.c
index 3c8677f..2ce80d9 100644
--- a/drivers/gpu/drm/radeon/r520.c
+++ b/drivers/gpu/drm/radeon/r520.c
@@ -79,8 +79,8 @@
 		WREG32(0x4128, 0xFF);
 	}
 	r420_pipes_init(rdev);
-	gb_pipe_select = RREG32(0x402C);
-	tmp = RREG32(0x170C);
+	gb_pipe_select = RREG32(R400_GB_PIPE_SELECT);
+	tmp = RREG32(R300_DST_PIPE_CONFIG);
 	pipe_select_current = (tmp >> 2) & 3;
 	tmp = (1 << pipe_select_current) |
 	      (((gb_pipe_select >> 8) & 0xF) << 4);