drm/amd/display: Add writeback enable field (wb_enabled)

[WHAT]
Add a new field to keep track whether a crtc is previously
writeback-enabled.

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>