drm/pl111: Fix module probe bug

Commit a30933c27602 ("drm/pl111: Support the Versatile Express")
Added a second module using the builtin_platform_driver() call,
which works fine as long as you do not try to build the PL111
driver as a module, because a module can only have one initcall
and cause the following build bug:

(...) multiple definition of `init_module' (...)

Reported-by: Daniel Vetter <[e-mail]>
Cc: Liviu Dudau <[e-mail]>
Cc: Pawel Moll <[e-mail]>
Cc: Eric Anholt <[e-mail]>
Cc: Robin Murphy <[e-mail]>
Reviewed-by: Sean Paul <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Fixes: a30933c27602 ("drm/pl111: Support the Versatile Express")
Signed-off-by: Linus Walleij <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/pl111/pl111_versatile.c

···
 	if (versatile_clcd_type == VEXPRESS_CLCD_V2M) {
 		struct platform_device *pdev;
 
+		/* Registers a driver for the muxfpga */
+		ret = vexpress_muxfpga_init();
+		if (ret) {
+			dev_err(dev, "unable to initialize muxfpga driver\n");
+			return ret;
+		}
+
 		/* Call into deep Vexpress configuration API */
 		pdev = of_find_device_by_node(np);
 		if (!pdev) {

drivers/gpu/drm/pl111/pl111_vexpress.c

···
 	.probe = vexpress_muxfpga_probe,
 };
 
-builtin_platform_driver(vexpress_muxfpga_driver);
+int vexpress_muxfpga_init(void)
+{
+	int ret;
+
+	ret = platform_driver_register(&vexpress_muxfpga_driver);
+	/* -EBUSY just means this driver is already registered */
+	if (ret == -EBUSY)
+		ret = 0;
+	return ret;
+}

drivers/gpu/drm/pl111/pl111_vexpress.h

···
 			     struct pl111_drm_dev_private *priv,
 			     struct regmap *map);
 
+int vexpress_muxfpga_init(void);
+
 #else
 
 static inline int pl111_vexpress_clcd_init(struct device *dev,
···
 					   struct regmap *map)
 {
 	return -ENODEV;
+}
+
+static inline int vexpress_muxfpga_init(void)
+{
+	return 0;
 }
 
 #endif