drm/amd/display: Fix writeback_info never got updated

[WHY]
wb_enabled field is set to false before it is used, and the following
code will never be executed.

[HOW]
Setting wb_enable to false after all removal work is completed.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>