drm/ast: Enable atomic modesetting

This commit sets the remaining atomic-modesetting helpers and the flag
DRIVER_ATOMIC. Legacy cursor functions are removed in favor of the cursor
plane. For power management, atomic helpers replace the indvidual
operations that the driver currently runs.

Atomic modesetting is enabled with this commit.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-10-tzimmermann@suse.de