sdm: drm: Add null check for DRM Atomic Interface

When the device boots up after a crash, there can be a race condition
between DRM Atomic interface creation and PowerOn from SurfaceFlinger.
Add null check in Suspend and Resume APIs to avoid framework reboot in
such cases.

Change-Id: I353b1e0c8cc61c93eb7939a78d2634069488e8fd
1 file changed