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

[ Upstream commit 50151b7f ]

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>