drm/amdgpu: user amdgpu_bo_create_kernel for the UVD BO

Saves us some code.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>