drm/ast: Initialize planes in helper functions

This change will help with inlining cursor functions into modesetting
code. The primary plane's field used to be cleared with memset(). This
has been dropped as the memory is always allocated with kzalloc().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-4-tzimmermann@suse.de