drm/ast: Don't set struct drm_crtc_state.no_vblank explicitly

As ast does not initialize vblanking, atomic helpers initialize the
value of struct drm_crtc_state.no_vblank to be true. No need to set
it from within the driver.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200129120531.6891-4-tzimmermann@suse.de