drm/amd/display: Disable link before reenable

[Why]
Black screen seen after display is disabled then re-enabled.
Caused by difference in link settings when
switching between different resolutions.

[How]
In PnP case, or whenever the display is
still enabled but the driver is unloaded,
disable link before re-enabling with new link settings.

Signed-off-by: Lucy Li <lucy.li@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>