drm/amd/display: Add user backlight level reg write

[Why]
Porting abm from dmcu to dmcub missed one register write.

[How]
Add this register write in the SetBacklightLevel sequence.

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>