msm_fb: Register frame buffer device for device shutdown

Many display panels have a requirement of executing panel-off
commands when the device goes to shutdown. Hence, register frame
buffer device for device shutdown so that the panel-off commands
can be executed successfully.

Change-Id: If39a93247542f5bbee8b0c4d1df1aab13270aa06
Signed-off-by: raghavendra ambadas <rambad@codeaurora.org>