drm/ast: Fix soft lockup

In the Linux kernel, the following vulnerability has been resolved:

drm/ast: Fix soft lockup

There is a while-loop in ast_dp_set_on_off() that could lead to

infinite-loop. This is because the register, VGACRI-Dx, checked in

this API is a scratch register actually controlled by a MCU, named

DPMCU, in BMC.

These scratch registers are protected by scu-lock. If suc-lock is not

off, DPMCU can not update these registers and then host will have soft

lockup due to never updated status.

DPMCU is used to control DP and relative registers to handshake with

host's VGA driver. Even the most time-consuming task, DP's link

training, is less than 100ms. 200ms should be enough.