drm/amd/display: Avoid NULL pointer in set_backlight when ABM is NULL

[Why]
On ASIC without ABM support (most dGPU) we run into a null pointer
dereference when attempting to set the backlight level.

[How]
This function requires ABM, so fix up the condition to only allow
DMCU to be optional.

Signed-off-by: Nicholas Kazlauskas <[e-mail]>
Reviewed-by: Wyatt Wood <[e-mail]>
Acked-by: Rodrigo Siqueira <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

···
 	int i;
 	DC_LOGGER_INIT(link->ctx->logger);
 
-	if ((dmcu == NULL && abm == NULL) ||
-		(abm->funcs->set_backlight_level_pwm == NULL))
+	if (abm == NULL || (abm->funcs->set_backlight_level_pwm == NULL))
 		return false;
 
 	if (dmcu)