drm/pl111: fix warnings without CONFIG_ARM_AMBA

The driver is written in a way to enable compile-testing without CONFIG_ARM_AMBA,
but it just causes needless warnings:

drivers/gpu/drm/pl111/pl111_drv.c:149:26: error: 'pl111_drm_driver' defined but not used [-Werror=unused-variable]
drivers/gpu/drm/pl111/pl111_drv.c:81:12: error: 'pl111_modeset_init' defined but not used [-Werror=unused-function]

This unhides the probe/remove functions again and just leaves the driver
object as unused when CONFIG_ARM_AMBA is disabled, with a __maybe_unused
annotation to shut up the warning.

Fixes: bed41005 ("drm/pl111: Initial drm/kms driver for pl111")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524155020.1777369-1-arnd@arndb.de

drivers/gpu/drm/pl111/pl111_drv.c

@@ -179,7 +179,6 @@ static struct drm_driver pl111_drm_driver = {
 #endif
 };
 
-#ifdef CONFIG_ARM_AMBA
 static int pl111_amba_probe(struct amba_device *amba_dev,
 			    const struct amba_id *id)
 {
@@ -252,7 +251,7 @@ static struct amba_id pl111_id_table[] = {
 	{0, 0},
 };
 
-static struct amba_driver pl111_amba_driver = {
+static struct amba_driver pl111_amba_driver __maybe_unused = {
 	.drv = {
 		.name = "drm-clcd-pl111",
 	},
@@ -261,8 +260,9 @@ static struct amba_driver pl111_amba_driver = {
 	.id_table = pl111_id_table,
 };
 
+#ifdef CONFIG_ARM_AMBA
 module_amba_driver(pl111_amba_driver);
-#endif /* CONFIG_ARM_AMBA */
+#endif
 
 MODULE_DESCRIPTION(DRIVER_DESC);
 MODULE_AUTHOR("ARM Ltd.");