msm_fb: mdss: register framebuffer devices for device shutdown

Many display panels mandate that when device is powered down, it is
necessary to execute panel power-off sequences. Register framebuffer
devices for shutdown event so that power-off sequences of display panel
can be executed.

Change-Id: I1adbd38affa83a2f8887f8dab90907b6465de3cd
CRs-Fixed: 517260
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>