drm/exynos: Call drm_atomic_helper_shutdown() at shutdown/unbind time

Based on grepping through the source code this driver appears to be
missing a call to drm_atomic_helper_shutdown() at system shutdown time
and at driver unbind time. Among other things, this means that if a
panel is in use that it won't be cleanly powered off at system
shutdown time.

The fact that we should call drm_atomic_helper_shutdown() in the case
of OS shutdown/restart and at driver remove (or unbind) time comes
straight out of the kernel doc "driver instance overview" in
drm_drv.c.

A few notes about this fix:
- When adding drm_atomic_helper_shutdown() to the unbind path, I added
  it after drm_kms_helper_poll_fini() since that's when other drivers
  seemed to have it.
- Technically with a previous patch, ("drm/atomic-helper:
  drm_atomic_helper_shutdown(NULL) should be a noop"), we don't
  actually need to check to see if our "drm" pointer is NULL before
  calling drm_atomic_helper_shutdown(). We'll leave the "if" test in,
  though, so that this patch can land without any dependencies. It
  could potentially be removed later.
- This patch also makes sure to set the drvdata to NULL in the case of
  bind errors to make sure that shutdown can't access freed data.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>