drm/msm: add shutdown support for display platform_driver

Define shutdown callback for display drm driver,
so as to disable all the CRTCS when shutdown
notification is received by the driver.

This change will turn off the timing engine so
that no display transactions are requested
while mmu translations are getting disabled
during reboot sequence.

Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>

Changes in v2:
	- Remove NULL check from msm_pdev_shutdown (Stephen Boyd)
	- Change commit text to reflect when this issue
	  was uncovered (Sai Prakash Ranjan)

Signed-off-by: Rob Clark <robdclark@chromium.org>