drm/amd/display: Add writeback enable/disable in dc

[WHAT]
The enable and disable writeback calls need to be included in the
coressponding functions in dc_stream.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
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>