drm/amd/amdgpu: Code comments for the amdgpu_ttm.c driver. (v2)

NFC just comments.

(v2):  Updated based on feedback from Alex Deucher.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>