drm/msm: Use drm_atomic_helper_shutdown

drm_plane_helper_disable is a non-atomic drivers only function, and
will blow up (since no one passes the locking context it needs).

Atomic drivers which want to quiescent their hw on unload should
use drm_atomic_helper_shutdown() instead.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Rajesh Yadav <ryadav@codeaurora.org>
Cc: Chandan Uddaraju <chandanu@codeaurora.org>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Jeykumar Sankaran <jsanka@codeaurora.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Gustavo Padovan <gustavo.padovan@collabora.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-12-daniel.vetter@ffwll.ch