drm/fb-helper: set x/yres_virtual in drm_fb_helper_check_var

Drivers are supposed to fix this up if needed if they don't outright
reject it. Uncovered by 6c11df5 ("fbmem: Check virtual screen
sizes in fb_set_var()").

Reported-by: syzbot+20dcf81733d43ddff661@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=c5faf983bfa4a607de530cd3bb008888bf06cefc
Cc: stable@vger.kernel.org # v5.4+
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404194038.472803-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_fb_helper.c

@@ -1589,6 +1589,9 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
 		return -EINVAL;
 	}
 
+	var->xres_virtual = fb->width;
+	var->yres_virtual = fb->height;
+
 	/*
 	 * Workaround for SDL 1.2, which is known to be setting all pixel format
 	 * fields values to zero in some cases. We treat this situation as a