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>