video:tegra:dc: fix hdcp hotplug issue

video:tegra:dc: fix hdcp hotplug issue

bit WRITE16 of HDMI_NV_PDISP_KEY_CTRL_0 shall be polled until it
reports DONE, which is value 0 to ensure the write is complete.

bug 858744
bug 861719

(cherry picked from commit d37336f3965cd1071afb6b03b979b0409ee480f1)
(reviewed on http://git-master/r/49821)

Change-Id: I38fe861a265db7d969f3a15f164724294d627cfd
Reviewed-on: http://git-master/r/52852
Tested-by: Ken Chang <kenc@nvidia.com>
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Rebase-Id: Rde95d37c2dfa5d1a169a3aa2a80f304f25b94c03