drm/amd/pm: Fix the null pointer dereference for vega10_hwmgr

commit 50151b7f upstream.

Check return value and conduct null pointer handling to avoid null pointer dereference.

Signed-off-by: Bob Zhou <bob.zhou@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Mukul Sikka <mukul.sikka@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>