drm/ast: Program display mode in CRTC's atomic_enable()

This change simplifies ast's modesetting code. The display mode
is now programmed from within the CRTC's atomic_enable(), which
only runs if we actually want to program the mode.

Corresponding code in atomic_flush() is being removed. Also removed
is atomic_begin(), which serves no purpose at all.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200914072236.19398-4-tzimmermann@suse.de