drm/atomic: Add missing drm_atomic_state_clear to atomic_remove_fb

All atomic state should be cleared when drm_modeset_backoff() is
called, because it drops all locks and the state becomes invalid.

The call to drm_atomic_state_clear was missing in atomic_remove_fb,
so add the missing call there.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170629115954.26029-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: db8f6403 ("drm: Convert drm_framebuffer_remove to atomic, v4.")
Cc: stable@vger.kernel.org # v4.12-rc1+