drm/amd/display: Check DMCU Exists Before Loading

Signed-off-by: Jerry (Fangzhi) Zuo <[e-mail]>
Reviewed-by: Hersen Wu <[e-mail]>
Acked-by: Aurabindo Pillai <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

···
 	struct dmcu_iram_parameters params;
 	unsigned int linear_lut[16];
 	int i;
-	struct dmcu *dmcu = adev->dm.dc->res_pool->dmcu;
+	struct dmcu *dmcu = NULL;
 	bool ret = false;
+
+	if (!adev->dm.fw_dmcu)
+		return detect_mst_link_for_all_connectors(adev->ddev);
+
+	dmcu = adev->dm.dc->res_pool->dmcu;
 
 	for (i = 0; i < 16; i++)
 		linear_lut[i] = 0xFFFF * i / 15;