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>

drivers/gpu/drm/amd/display/dc/dce/dmub_abm.c

@@ -136,6 +136,8 @@ static void dmcub_set_backlight_level(
 
 	dmub_abm_set_pipe(&dce_abm->base, otg_inst);
 
+	REG_UPDATE(BL1_PWM_USER_LEVEL, BL1_PWM_USER_LEVEL, backlight_pwm_u16_16);
+
 	if (otg_inst == 0)
 		frame_ramp = 0;