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: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-3-tzimmermann@suse.de