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: [email protected]
Link: https://syzkaller.appspot.com/bug?id=c5faf983bfa4a607de530cd3bb008888bf06cefc
Cc: [email protected] # v5.4+
Cc: Daniel Vetter <[email protected]>
Cc: Javier Martinez Canillas <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Reviewed-by: Javier Martinez Canillas <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: danvet

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