drm/ast: Set format registers in primary plane's update

The atomic modesetting code tried to distinguish format changes from
full modesetting operations. But the implementation was buggy and the
format registers were often updated even for simple pageflips.

Fix this problem by handling format changes in the primary plane's
update function.

v3:
	* program format in primary plane's update function

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200914072236.19398-2-tzimmermann@suse.de