drm/ast: Replace struct_drm_device.dev_private with to_ast_private()

The ast code still references dev_private in several place when looking
up the ast device structure. Convert the remaining locations to use
to_ast_private().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-5-tzimmermann@suse.de