drm/amdgpu: use GTT for uvd_get_create/destory_msg

On modern gpus, GTT (system memory) works as well here, and this may
also be a workaround for platforms which cannot map vram correctly.

Signed-off-by: chenli <chenli@uniontech.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>