drm_atomic_helper_commit_duplicated_state() sets state->acquire_ctx to
the context given in the argument and leaves it in state after it
quits. The lifetime of state and context are not guaranteed to be the
same, so we shouldn't leave that pointer hanging around. This patch
resets the context to NULL to avoid any oopses.

Changes in v2:
- Added to the set
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129150423.239081-1-sean@poorly.run