drm/fb-helper: Fix clip rectangle height

Computing the clip rectangle is prone to off-by-one errors when writes
happen near the end of a memory page. Point the end of the memory area
to the first trailing byte, so that (end - start) returns the area's
length.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209161617.3553-2-tzimmermann@suse.de

drivers/gpu/drm/drm_fb_helper.c

@@ -699,7 +699,7 @@ void drm_fb_helper_deferred_io(struct fb_info *info,
 	max = 0;
 	list_for_each_entry(page, pagelist, lru) {
 		start = page->index << PAGE_SHIFT;
-		end = start + PAGE_SIZE - 1;
+		end = start + PAGE_SIZE;
 		min = min(min, start);
 		max = max(max, end);
 	}