drm/amd/display: Hande writeback request from userspace

[WHAT]
Handle writeback requests and fill in the required information for DWB
programming and setup.

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>