drm/ast: Rename struct ast_cursor_plane to struct ast_plane

Rename the plane structure struct ast_cursor_plane to struct
ast_plane as it will be used for the primary plane as well. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013112923.769-6-tzimmermann@suse.de