drm/amdgpu: Move null pointer dereference check

Null pointer dereference check should have been checked,
ahead of below routine.
	struct amdgpu_device *adev = hwmgr->adev;

With this commit, it could avoid potential NULL dereference.

Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>