drm/ast: Don't clear base address and offset with default values

The content of the base-address and offset registers are state of
the primary plane. Clearing it to default values will interfere with
plane functions for atomic mode setting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-4-tzimmermann@suse.de