Add missing acronyms to the amdgppu glossary.

Closes: drm/amd#1939


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tales Lelo da Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>