drm/ast: Add CRTC helpers for atomic modesetting

As the CRTC code has already been prepared for a split between mode
setting and plane handling, most of the CRTC's atomic modesetting is
build upon primitives of the non-atomic implementation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-8-tzimmermann@suse.de