drm/amdgpu: handle amdgpu_cgs_create_device() errors in amd_powerplay_create()

Add error handling to propagate amdgpu_cgs_create_device() failures
to the caller. When amdgpu_cgs_create_device() fails, release hwmgr
and return -ENOMEM to prevent null pointer dereference.

[v1]->[v2]: Change error code from -EINVAL to -ENOMEM. Free hwmgr.

Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>