drm/ast: Upcast from DRM device to ast structure via to_ast_private()

All upcasting from struct drm_device to struct ast_private is now
performed via to_ast_private(). Using struct drm_device.dev_private
is deprecated. The ast variable in ast_crtc_helper_atomic_check() is
unused, so removed it.

v2:
	* fix typo in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200617080340.29584-4-tzimmermann@suse.de