drm/debugfs: Fix framebuffer debugfs file init

The introduction of: drm/framebuffer: Add framebuffer debugfs file
broke vgem. That patch assumed that all drivers had initialized the
dev->mode_config.fb_lock mutex which happens in drm_mode_config_init().
vgem doesn't need to call drm_mode_config_init().

Fix this by only creating the framebuffer debugfs file for modesetting
drivers.

Fixes: 45d58b40 ("drm/framebuffer: Add framebuffer debugfs file")
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113231201.26177-1-noralf@tronnes.org

drivers/gpu/drm/drm_debugfs.c

@@ -158,11 +158,13 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
 		}
 	}
 
+	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
 		ret = drm_framebuffer_debugfs_init(minor);
 		if (ret) {
 			DRM_ERROR("Failed to create framebuffer debugfs file\n");
 			return ret;
 		}
+	}
 
 	if (dev->driver->debugfs_init) {
 		ret = dev->driver->debugfs_init(minor);