drm/msm: remove shutdown callback from msm_platform_driver

drm/msm: remove shutdown callback from msm_platform_driver

The msm_drv_shutdown only makes sense for the KMS-enabled devices, while
msm_platform_driver is only used in the headless case. Remove the
shutdown callback from the driver structure.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/561644/
Signed-off-by: Rob Clark <robdclark@chromium.org>