drm/amdgpu: add GART, GPUVM, and GTT to glossary

Add definitions to clarify GPU virtual memory.

v2: clarify the terms a bit more

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Suggested-by: Peter Maucher <bellosilicio@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20221201214153.8453-1-alexander.deucher@amd.com