drm/ast: Fix invalid usage of AST_MAX_HWC_WIDTH in cursor atomic_check

Use AST_MAX_HWC_HEIGHT for setting offset_y in the cursor plane's
atomic_check. The code used AST_MAX_HWC_WIDTH instead. This worked
because both constants has the same value.
Signed-off-by: NThomas Zimmermann <tzimmermann@suse.de>
Acked-by: NGerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-3-tzimmermann@suse.de

drivers/gpu/drm/ast/ast_mode.c

@@ -688,7 +688,7 @@ ast_cursor_plane_helper_atomic_update(struct drm_plane *plane,
 	unsigned int offset_x, offset_y;
 
 	offset_x = AST_MAX_HWC_WIDTH - fb->width;
-	offset_y = AST_MAX_HWC_WIDTH - fb->height;
+	offset_y = AST_MAX_HWC_HEIGHT - fb->height;
 
 	if (state->fb != old_state->fb) {
 		/* A new cursor image was installed. */