drm/amdgpu: update documentation for amdgpu_drv.c

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>