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: 45d58b40292b ("drm/framebuffer: Add framebuffer debugfs file")
Suggested-by: Chris Wilson <[e-mail]>
Signed-off-by: Noralf Trønnes <[e-mail]>
Reviewed-by: Dave Airlie <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_debugfs.c

···
 		}
 	}
 
-	ret = drm_framebuffer_debugfs_init(minor);
-	if (ret) {
-		DRM_ERROR("Failed to create framebuffer debugfs file\n");
-		return ret;
+	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) {