mdss: displayport: fix shutdown sequence

It is recommended to ensure that the interface timing engine is on
when configuring the hardware to switch to "IDLE pattern" state
as part of the shutdown sequence. In addition, it is also preferred
to issue a global software reset of the controller to ensure that
it's state machine is reset for any subsequent connections.
This updated sequence fixes link training failures seen during
multiple connect/disconnect use-cases.

Change-Id: I1984c1fc8c3e4a5f9c818240ec7e0323a68bfe3b
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>