drm/amdgpu: add amdgpu uapi header (v4)

This header defines the ioctl interface to the driver.

v2: remove stale tiling defines
v3: add appropriate padding
v4: remove executable bits on header

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>