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/r420.c b/drivers/gpu/drm/radeon/r420.c
index c387346..0b59ed7 100644
--- a/drivers/gpu/drm/radeon/r420.c
+++ b/drivers/gpu/drm/radeon/r420.c
@@ -96,7 +96,7 @@
 		       "programming pipes. Bad things might happen.\n");
 	}
 	/* get max number of pipes */
-	gb_pipe_select = RREG32(0x402C);
+	gb_pipe_select = RREG32(R400_GB_PIPE_SELECT);
 	num_pipes = ((gb_pipe_select >> 12) & 3) + 1;
 
 	/* SE chips have 1 pipe */