drm/ast: Introduce struct ast_crtc_state

AST-specific CRTC state can be placed in the new struct ast_crtc_state.
The atomic check functions of the CRTC and the primary plane will store
the VBIOS mode info and the framebuffer format here. The CRTC will consume
these during atomic_flush().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202111557.15176-6-tzimmermann@suse.de