drm/ast: Enable and disable screen in primary-plane functions

Enabling and disabling the screen used to be done in the register
initialization and the DPMS function. None of these places is related
to the screen's output.

Now the primary plane's update and disable functions handle screen
display state. The primary plane can now be switched off without
displaying garbage.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202111557.15176-3-tzimmermann@suse.de