drm/amdgpu: add UAPI for creating encrypted buffers

Add a flag to the GEM_CREATE ioctl to create encrypted buffers.
Buffers with this flag set will be created with the TMZ bit set
in the PTEs or engines accessing them.  This is required in order
to properly access the data from the engines.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>