drm/amdgpu: fix coding style and documentation in amdgpu_gtt_mgr.c

Avoid the forward define, fix coding style, add documentation.

No functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.aiemd@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>