drm/radeon: cleanup properly if mmio mapping fails

If we fail to map the mmio BAR, skip driver tear down
that requires mmio.

Should fix:
https://bugzilla.kernel.org/show_bug.cgi?id=56541

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/radeon_kms.c

@@ -50,9 +50,13 @@ int radeon_driver_unload_kms(struct drm_device *dev)
 
 	if (rdev == NULL)
 		return 0;
+	if (rdev->rmmio == NULL)
+		goto done_free;
 	radeon_acpi_fini(rdev);
 	radeon_modeset_fini(rdev);
 	radeon_device_fini(rdev);
+
+done_free:
 	kfree(rdev);
 	dev->dev_private = NULL;
 	return 0;