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.

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Fixes: 594e9c04

 ("drm/ast: Create the driver for ASPEED proprietory Display-Port")
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.19+
Link: https://patchwork.freedesktop.org/patch/msgid/20240403090246.1495487-1-jammy_huang@aspeedtech.com