drm/ast: Move cursor functions to ast_cursor.c

The cursor manipulation functions are unrelated to modesetting. Move
them into their own file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-2-tzimmermann@suse.de