drm/amd/powerplay: fix missed hwmgr check warning before call gfx_off_control handler

Patch 9667849b: "drm/amd/powerplay: add control gfxoff enabling in late
init" from Mar 13, 2018, leads to the following static checker warning:

	drivers/gpu/drm/amd/amdgpu/../powerplay/amd_powerplay.c:194
pp_late_init()
	error: we previously assumed 'hwmgr' could be null (see line 185)

drivers/gpu/drm/amd/amdgpu/../powerplay/amd_powerplay.c

This patch fixes the warning to add hwmgr checking.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/powerplay/amd_powerplay.c

@@ -191,7 +191,8 @@ static int pp_late_init(void *handle)
 	if (adev->pm.smu_prv_buffer_size != 0)
 		pp_reserve_vram_for_smu(adev);
 
-	if (hwmgr->hwmgr_func->gfx_off_control &&
+	if (hwmgr && hwmgr->hwmgr_func &&
+	    hwmgr->hwmgr_func->gfx_off_control &&
 	    (hwmgr->feature_mask & PP_GFXOFF_MASK)) {
 		ret = hwmgr->hwmgr_func->gfx_off_control(hwmgr, true);
 		if (ret)