drm/ast: Store VBIOS mode info in struct ast_crtc_state

After looking up VBIOS mode information in CRTC's atomic_check(), we
can reuse it in atomic_flush(). No need for a second look-up.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202111557.15176-7-tzimmermann@suse.de