msm_fb: mdss: register mdss devices for system shutdown

When device is restarted it is necessary to power down display panel
in order to avoid any residue gliches for the next session. And in order
to power down display devices, respective drivers need to support
shutdown sequence. Implement driver shutdown for fb and mdp devices.

CRs-Fixed: 504751
Change-Id: I80acbdbb8eb9bb4999617698f7314fd8482f4403
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>