drm/ast: Store primary-plane format in struct ast_crtc_state

drm/ast: Store primary-plane format in struct ast_crtc_state

Reading the primary plane's framebuffer from the CRTC's atomic_flush()
function is fragile as the plane state or framebuffer can be NULL.
Instead, we let the plane's atomic_check() store the framebuffer format
in the CRTC state. The CRTC always receives the framebuffer format that
is currently programmed, or NULL if no mode has been set yet.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202111557.15176-8-tzimmermann@suse.de