drm/amd/display: Device flash garbage before get in OS

[Why]
Enabling stream with tg lock makes config settings
pending causing the garbage until tg unlock.

[How]
Keep the original lock mechanism
The driver doesn't lock tg if plane_state is null.

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Chiawen Huang <chiawen.huang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>