drm/amdgpu: fix the wrong type of gem object creation

We still use legacy type of gem_object_create, it should update to ttm_bo_type
now.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>