drm/amd/display: Add SetBacklight call to abm on dmcub

[Why]
Set backlight calls to firmware are are being prevented by dmcu == null
check. Dmcu is expected to be null in this case.

[How]
Only prevent call if dmcu and abm are null.  Also rename variable
'use_smooth_brightness' to 'fw_set_brightness' as it's more appropriate.

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>